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‎README.md‎

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| # | Title | Solution | Difficulty |
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| :-: | - | - | :-: |
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| <span id="1444">1444</span> | [Number of Ways of Cutting a Pizza](https://leetcode.com/problems/number-of-ways-of-cutting-a-pizza "切披萨的方案数") | [Go](problems/number-of-ways-of-cutting-a-pizza) | Hard |
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| <span id="1443">1443</span> | [Minimum Time to Collect All Apples in a Tree](https://leetcode.com/problems/minimum-time-to-collect-all-apples-in-a-tree "收集树上所有苹果的最少时间") | [Go](problems/minimum-time-to-collect-all-apples-in-a-tree) | Medium |
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| <span id="1442">1442</span> | [Count Triplets That Can Form Two Arrays of Equal XOR](https://leetcode.com/problems/count-triplets-that-can-form-two-arrays-of-equal-xor "形成两个异或相等数组的三元组数目") | [Go](problems/count-triplets-that-can-form-two-arrays-of-equal-xor) | Medium |
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| <span id="1441">1441</span> | [Build an Array With Stack Operations](https://leetcode.com/problems/build-an-array-with-stack-operations "用栈操作构建数组") | [Go](problems/build-an-array-with-stack-operations) | Easy |
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| <span id="1440">1440</span> | [Evaluate Boolean Expression](https://leetcode.com/problems/evaluate-boolean-expression) 🔒 | [MySQL](problems/evaluate-boolean-expression) | Medium |
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| <span id="1439">1439</span> | [Find the Kth Smallest Sum of a Matrix With Sorted Rows](https://leetcode.com/problems/find-the-kth-smallest-sum-of-a-matrix-with-sorted-rows "有序矩阵中的第 k 个最小数组和") | [Go](problems/find-the-kth-smallest-sum-of-a-matrix-with-sorted-rows) | Hard |
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| <span id="1438">1438</span> | [Longest Continuous Subarray With Absolute Diff Less Than or Equal to Limit](https://leetcode.com/problems/longest-continuous-subarray-with-absolute-diff-less-than-or-equal-to-limit "绝对差不超过限制的最长连续子数组") | [Go](problems/longest-continuous-subarray-with-absolute-diff-less-than-or-equal-to-limit) | Medium |
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| <span id="1437">1437</span> | [Check If All 1's Are at Least Length K Places Away](https://leetcode.com/problems/check-if-all-1s-are-at-least-length-k-places-away "是否所有 1 都至少相隔 k 个元素") | [Go](problems/check-if-all-1s-are-at-least-length-k-places-away) | Medium |

‎problems/build-an-array-with-stack-operations/README.md‎

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<!--|This file generated by command(leetcode description); DO NOT EDIT. |-->
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<!--+----------------------------------------------------------------------+-->
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<!--|@author openset <[email protected]> |-->
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<!--|@link https://github.com/openset |-->
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<!--|@home https://github.com/openset/leetcode |-->
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<!--+----------------------------------------------------------------------+-->
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[< Previous](../evaluate-boolean-expression "Evaluate Boolean Expression")
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[Next >](../count-triplets-that-can-form-two-arrays-of-equal-xor "Count Triplets That Can Form Two Arrays of Equal XOR")
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## [1441. Build an Array With Stack Operations (Easy)](https://leetcode.com/problems/build-an-array-with-stack-operations "用栈操作构建数组")
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<p>Given an array <code>target</code> and&nbsp;an integer <code>n</code>. In each iteration, you will read a number from &nbsp;<code>list = {1,2,3..., n}</code>.</p>
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<p>Build the <code>target</code>&nbsp;array&nbsp;using the following operations:</p>
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<ul>
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<li><strong>Push</strong>: Read a new element from the beginning&nbsp;<code>list</code>, and push it in the array.</li>
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<li><strong>Pop</strong>: delete the last element of&nbsp;the array.</li>
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<li>If the target array is already&nbsp;built, stop reading more elements.</li>
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</ul>
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<p>You are guaranteed that the target array is strictly&nbsp;increasing, only containing&nbsp;numbers between 1 to <code>n</code>&nbsp;inclusive.</p>
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<p>Return the operations to build the target array.</p>
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<p>You are guaranteed that the answer is unique.</p>
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<p>&nbsp;</p>
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<p><strong>Example 1:</strong></p>
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<pre>
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<strong>Input:</strong> target = [1,3], n = 3
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<strong>Output:</strong> [&quot;Push&quot;,&quot;Push&quot;,&quot;Pop&quot;,&quot;Push&quot;]
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<strong>Explanation:
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</strong>Read number 1 and automatically push in the array -&gt; [1]
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Read number 2 and automatically push in the array then Pop it -&gt; [1]
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Read number 3 and automatically push in the array -&gt; [1,3]
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</pre>
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<p><strong>Example 2:</strong></p>
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<pre>
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<strong>Input:</strong> target = [1,2,3], n = 3
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<strong>Output:</strong> [&quot;Push&quot;,&quot;Push&quot;,&quot;Push&quot;]
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</pre>
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<p><strong>Example 3:</strong></p>
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<pre>
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<strong>Input:</strong> target = [1,2], n = 4
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<strong>Output:</strong> [&quot;Push&quot;,&quot;Push&quot;]
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<strong>Explanation: </strong>You only need to read the first 2 numbers and stop.
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</pre>
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<p><strong>Example 4:</strong></p>
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<pre>
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<strong>Input:</strong> target = [2,3,4], n = 4
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<strong>Output:</strong> [&quot;Push&quot;,&quot;Pop&quot;,&quot;Push&quot;,&quot;Push&quot;,&quot;Push&quot;]
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</pre>
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<p>&nbsp;</p>
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<p><strong>Constraints:</strong></p>
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<ul>
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<li><code>1 &lt;= target.length &lt;= 100</code></li>
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<li><code>1 &lt;= target[i]&nbsp;&lt;= 100</code></li>
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<li><code>1 &lt;= n &lt;= 100</code></li>
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<li><code>target</code> is strictly&nbsp;increasing.</li>
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</ul>
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### Related Topics
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[[Stack](../../tag/stack/README.md)]
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### Hints
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<details>
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<summary>Hint 1</summary>
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Use “Push” for numbers to be kept in target array and [“Push”, “Pop”] for numbers to be discarded.
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</details>

‎problems/count-triplets-that-can-form-two-arrays-of-equal-xor/README.md‎

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<!--|This file generated by command(leetcode description); DO NOT EDIT. |-->
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<!--+----------------------------------------------------------------------+-->
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<!--|@author openset <[email protected]> |-->
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<!--|@link https://github.com/openset |-->
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<!--|@home https://github.com/openset/leetcode |-->
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<!--+----------------------------------------------------------------------+-->
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[< Previous](../build-an-array-with-stack-operations "Build an Array With Stack Operations")
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[Next >](../minimum-time-to-collect-all-apples-in-a-tree "Minimum Time to Collect All Apples in a Tree")
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## [1442. Count Triplets That Can Form Two Arrays of Equal XOR (Medium)](https://leetcode.com/problems/count-triplets-that-can-form-two-arrays-of-equal-xor "形成两个异或相等数组的三元组数目")
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<p>Given an array of&nbsp;integers <code>arr</code>.</p>
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<p>We want to select three indices <code>i</code>, <code>j</code> and <code>k</code> where <code>(0 &lt;= i &lt; j &lt;= k &lt; arr.length)</code>.</p>
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<p>Let&#39;s define <code>a</code> and <code>b</code> as follows:</p>
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<ul>
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<li><code>a = arr[i] ^ arr[i + 1] ^ ... ^ arr[j - 1]</code></li>
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<li><code>b = arr[j] ^ arr[j + 1] ^ ... ^ arr[k]</code></li>
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</ul>
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<p>Note that <strong>^</strong> denotes the <strong>bitwise-xor</strong> operation.</p>
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<p>Return <em>the number of triplets</em> (<code>i</code>, <code>j</code> and <code>k</code>) Where <code>a == b</code>.</p>
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<p>&nbsp;</p>
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<p><strong>Example 1:</strong></p>
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<pre>
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<strong>Input:</strong> arr = [2,3,1,6,7]
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<strong>Output:</strong> 4
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<strong>Explanation:</strong> The triplets are (0,1,2), (0,2,2), (2,3,4) and (2,4,4)
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</pre>
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<p><strong>Example 2:</strong></p>
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<pre>
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<strong>Input:</strong> arr = [1,1,1,1,1]
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<strong>Output:</strong> 10
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</pre>
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<p><strong>Example 3:</strong></p>
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<pre>
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<strong>Input:</strong> arr = [2,3]
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<strong>Output:</strong> 0
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</pre>
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<p><strong>Example 4:</strong></p>
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<pre>
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<strong>Input:</strong> arr = [1,3,5,7,9]
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<strong>Output:</strong> 3
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</pre>
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<p><strong>Example 5:</strong></p>
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<pre>
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<strong>Input:</strong> arr = [7,11,12,9,5,2,7,17,22]
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<strong>Output:</strong> 8
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</pre>
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<p>&nbsp;</p>
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<p><strong>Constraints:</strong></p>
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<ul>
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<li><code>1 &lt;= arr.length &lt;= 300</code></li>
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<li><code>1 &lt;= arr[i] &lt;= 10^8</code></li>
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</ul>
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### Related Topics
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[[Bit Manipulation](../../tag/bit-manipulation/README.md)]
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[[Array](../../tag/array/README.md)]
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[[Math](../../tag/math/README.md)]
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### Hints
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<details>
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<summary>Hint 1</summary>
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We are searching for sub-array of length ≥ 2 and we need to split it to 2 non-empty arrays so that the xor of the first array is equal to the xor of the second array. This is equivalent to searching for sub-array with xor = 0.
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</details>
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<details>
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<summary>Hint 2</summary>
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Keep the prefix xor of arr in another array, check the xor of all sub-arrays in O(n^2), if the xor of sub-array of length x is 0 add x-1 to the answer.
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</details>

‎problems/evaluate-boolean-expression/README.md‎

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<!--|This file generated by command(leetcode description); DO NOT EDIT. |-->
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<!--|@author openset <[email protected]> |-->
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<!--|@link https://github.com/openset |-->
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<!--|@home https://github.com/openset/leetcode |-->
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<!--+----------------------------------------------------------------------+-->
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[< Previous](../find-the-kth-smallest-sum-of-a-matrix-with-sorted-rows "Find the Kth Smallest Sum of a Matrix With Sorted Rows")
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[Next >](../build-an-array-with-stack-operations "Build an Array With Stack Operations")
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## [1440. Evaluate Boolean Expression (Medium)](https://leetcode.com/problems/evaluate-boolean-expression "")
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‎problems/evaluate-boolean-expression/mysql_schemas.sql‎

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Create Table If Not Exists Variables (name varchar(3), value int);
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Create Table If Not Exists Expressions (left_operand varchar(3), operator ENUM('>', '<', '='), right_operand varchar(3));
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Truncate table Variables;
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insert into Variables (name, value) values ('x', '66');
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insert into Variables (name, value) values ('y', '77');
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Truncate table Expressions;
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insert into Expressions (left_operand, operator, right_operand) values ('x', '>', 'y');
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insert into Expressions (left_operand, operator, right_operand) values ('x', '<', 'y');
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insert into Expressions (left_operand, operator, right_operand) values ('x', '=', 'y');
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insert into Expressions (left_operand, operator, right_operand) values ('y', '>', 'x');
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insert into Expressions (left_operand, operator, right_operand) values ('y', '<', 'x');
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insert into Expressions (left_operand, operator, right_operand) values ('x', '=', 'x');

‎problems/find-k-closest-elements/README.md‎

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## [658. Find K Closest Elements (Medium)](https://leetcode.com/problems/find-k-closest-elements "找到 K 个最接近的元素")
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<p>
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Given a sorted array, two integers <code>k</code> and <code>x</code>, find the <code>k</code> closest elements to <code>x</code> in the array. The result should also be sorted in ascending order.
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If there is a tie, the smaller elements are always preferred.
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</p>
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<p><b>Example 1:</b><br />
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<pre>
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<b>Input:</b> [1,2,3,4,5], k=4, x=3
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<b>Output:</b> [1,2,3,4]
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</pre>
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</p>
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<p><b>Example 2:</b><br />
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<pre>
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<b>Input:</b> [1,2,3,4,5], k=4, x=-1
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<b>Output:</b> [1,2,3,4]
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<p>Given a sorted array <code>arr</code>, two integers <code>k</code> and <code>x</code>, find the <code>k</code> closest elements to <code>x</code> in the array. The result should also be sorted in ascending order. If there is a tie, the smaller elements are always preferred.</p>
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<p>&nbsp;</p>
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<p><strong>Example 1:</strong></p>
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<pre><strong>Input:</strong> arr = [1,2,3,4,5], k = 4, x = 3
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<strong>Output:</strong> [1,2,3,4]
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</pre><p><strong>Example 2:</strong></p>
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<pre><strong>Input:</strong> arr = [1,2,3,4,5], k = 4, x = -1
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<strong>Output:</strong> [1,2,3,4]
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</pre>
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</p>
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<p><b>Note:</b><br>
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<ol>
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<li>The value k is positive and will always be smaller than the length of the sorted array.</li>
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<li> Length of the given array is positive and will not exceed 10<sup>4</sup></li>
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<li> Absolute value of elements in the array and x will not exceed 10<sup>4</sup></li>
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</ol>
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</p>
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<hr />
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<p>
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<b><font color="red">UPDATE (2017/9/19):</font></b><br />
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The <i>arr</i> parameter had been changed to an <b>array of integers</b> (instead of a list of integers). <b><i>Please reload the code definition to get the latest changes</i></b>.
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</p>
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<p>&nbsp;</p>
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<p><strong>Constraints:</strong></p>
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<ul>
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<li><code>1 &lt;= k &lt;= arr.length</code></li>
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<li><code>1 &lt;= arr.length&nbsp;&lt;= 10^4</code></li>
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<li>Absolute value of elements in the array and <code>x</code> will not exceed 10<sup>4</sup></li>
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</ul>
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### Related Topics
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[[Binary Search](../../tag/binary-search/README.md)]

‎problems/find-the-kth-smallest-sum-of-a-matrix-with-sorted-rows/README.md‎

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[< Previous](../longest-continuous-subarray-with-absolute-diff-less-than-or-equal-to-limit "Longest Continuous Subarray With Absolute Diff Less Than or Equal to Limit")
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[Next >](../evaluate-boolean-expression "Evaluate Boolean Expression")
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## [1439. Find the Kth Smallest Sum of a Matrix With Sorted Rows (Hard)](https://leetcode.com/problems/find-the-kth-smallest-sum-of-a-matrix-with-sorted-rows "有序矩阵中的第 k 个最小数组和")
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‎problems/insert-into-a-binary-search-tree/README.md‎

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</pre>
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<p>&nbsp;</p>
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<p><strong>Constraints:</strong></p>
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<ul>
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<li>The number of nodes in the given tree will be between <code>0</code> and <code>10^4</code>.</li>
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<li>Each node will have a unique integer value from <code>0</code>&nbsp;to -<code>10^8</code>, inclusive.</li>
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<li><code>-10^8 &lt;= val &lt;= 10^8</code></li>
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<li>It&#39;s guaranteed that <code>val</code> does not exist in the original BST.</li>
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</ul>
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### Related Topics
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[[Tree](../../tag/tree/README.md)]
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‎problems/median-of-two-sorted-arrays/README.md‎

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[Next >](../longest-palindromic-substring "Longest Palindromic Substring")
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## [4. Median of Two Sorted Arrays (Hard)](https://leetcode.com/problems/median-of-two-sorted-arrays "寻找两个有序数组的中位数")
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## [4. Median of Two Sorted Arrays (Hard)](https://leetcode.com/problems/median-of-two-sorted-arrays "寻找两个正序数组的中位数")
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<p>There are two sorted arrays <b>nums1</b> and <b>nums2</b> of size m and n respectively.</p>
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‎problems/minimum-time-to-collect-all-apples-in-a-tree/README.md‎

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<!--|@author openset <[email protected]> |-->
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<!--|@link https://github.com/openset |-->
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<!--+----------------------------------------------------------------------+-->
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[< Previous](../count-triplets-that-can-form-two-arrays-of-equal-xor "Count Triplets That Can Form Two Arrays of Equal XOR")
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[Next >](../number-of-ways-of-cutting-a-pizza "Number of Ways of Cutting a Pizza")
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## [1443. Minimum Time to Collect All Apples in a Tree (Medium)](https://leetcode.com/problems/minimum-time-to-collect-all-apples-in-a-tree "收集树上所有苹果的最少时间")
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<p>Given an undirected tree consisting of <code>n</code> vertices numbered from 0 to <code>n-1</code>, which has some apples in their&nbsp;vertices. You spend 1 second to walk over one&nbsp;edge of the tree.&nbsp;<em>Return the minimum time in seconds&nbsp;you have to spend&nbsp;in order to collect all apples in the tree starting at <strong>vertex 0</strong> and coming back to this vertex.</em></p>
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<p>The edges of the undirected tree are given in the array <code>edges</code>, where <code>edges[i] = [from<sub>i</sub>, to<sub>i</sub>]</code> means that exists an edge connecting the vertices <code>from<sub>i</sub></code> and <code>to<sub>i</sub></code>. Additionally, there is&nbsp;a boolean array <code>hasApple</code>, where <code>hasApple[i] = true</code>&nbsp;means that&nbsp;vertex <code>i</code> has an apple, otherwise, it does not have any apple.</p>
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<p>&nbsp;</p>
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<p><strong>Example 1:</strong></p>
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<p><strong><img alt="" src="https://assets.leetcode.com/uploads/2020/04/23/min_time_collect_apple_1.png" style="width: 300px; height: 212px;" /></strong></p>
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<pre>
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<strong>Input:</strong> n = 7, edges = [[0,1],[0,2],[1,4],[1,5],[2,3],[2,6]], hasApple = [false,false,true,false,true,true,false]
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<strong>Output:</strong> 8
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<strong>Explanation:</strong> The figure above represents the given tree where red vertices have an apple. One optimal path to collect all apples is shown by the green arrows.
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</pre>
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<p><strong>Example 2:</strong></p>
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<p><strong><img alt="" src="https://assets.leetcode.com/uploads/2020/04/23/min_time_collect_apple_2.png" style="width: 300px; height: 212px;" /></strong></p>
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<pre>
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<strong>Input:</strong> n = 7, edges = [[0,1],[0,2],[1,4],[1,5],[2,3],[2,6]], hasApple = [false,false,true,false,false,true,false]
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<strong>Output:</strong> 6
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<strong>Explanation:</strong> The figure above represents the given tree where red vertices have an apple. One optimal path to collect all apples is shown by the green arrows.
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</pre>
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<p><strong>Example 3:</strong></p>
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<pre>
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<strong>Input:</strong> n = 7, edges = [[0,1],[0,2],[1,4],[1,5],[2,3],[2,6]], hasApple = [false,false,false,false,false,false,false]
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<strong>Output:</strong> 0
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</pre>
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<p>&nbsp;</p>
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<p><strong>Constraints:</strong></p>
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<ul>
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<li><code>1 &lt;= n &lt;= 10^5</code></li>
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<li><code>edges.length == n-1</code></li>
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<li><code>edges[i].length == 2</code></li>
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<li><code>0 &lt;= from<sub>i</sub>, to<sub>i</sub> &lt;= n-1</code></li>
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<li><code>from<sub>i</sub>&nbsp;&lt; to<sub>i</sub></code></li>
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<li><code>hasApple.length == n</code></li>
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</ul>
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### Related Topics
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[[Tree](../../tag/tree/README.md)]
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[[Depth-first Search](../../tag/depth-first-search/README.md)]
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### Hints
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<details>
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<summary>Hint 1</summary>
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Note that if a node u contains an apple then all edges in the path from the root to the node u have to be used forward and backward (2 times).
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</details>
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<details>
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<summary>Hint 2</summary>
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Therefore use a depth-first search (DFS) to check if an edge will be used or not.
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</details>

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