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‎README.md‎

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| # | Title | Solution | Difficulty |
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| <span id="1136">1136</span> | [Parallel Courses](https://leetcode.com/problems/parallel-courses "平行课程") 🔒 | [Go](https://github.com/openset/leetcode/tree/master/problems/parallel-courses) | Hard |
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| <span id="1135">1135</span> | [Connecting Cities With Minimum Cost](https://leetcode.com/problems/connecting-cities-with-minimum-cost "最低成本联通所有城市") 🔒 | [Go](https://github.com/openset/leetcode/tree/master/problems/connecting-cities-with-minimum-cost) | Medium |
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| <span id="1134">1134</span> | [Armstrong Number](https://leetcode.com/problems/armstrong-number "阿姆斯特朗数") 🔒 | [Go](https://github.com/openset/leetcode/tree/master/problems/armstrong-number) | Easy |
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| <span id="1133">1133</span> | [Largest Unique Number](https://leetcode.com/problems/largest-unique-number "最大唯一数") 🔒 | [Go](https://github.com/openset/leetcode/tree/master/problems/largest-unique-number) | Easy |
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| <span id="1132">1132</span> | [Reported Posts II](https://leetcode.com/problems/reported-posts-ii) 🔒 | [MySQL](https://github.com/openset/leetcode/tree/master/problems/reported-posts-ii) | Medium |
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| <span id="1131">1131</span> | [Maximum of Absolute Value Expression](https://leetcode.com/problems/maximum-of-absolute-value-expression "绝对值表达式的最大值") | [Go](https://github.com/openset/leetcode/tree/master/problems/maximum-of-absolute-value-expression) | Medium |
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| <span id="1130">1130</span> | [Minimum Cost Tree From Leaf Values](https://leetcode.com/problems/minimum-cost-tree-from-leaf-values "叶值的最小代价生成树") | [Go](https://github.com/openset/leetcode/tree/master/problems/minimum-cost-tree-from-leaf-values) | Medium |

‎problems/armstrong-number/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](https://github.com/openset/leetcode/tree/master/problems/largest-unique-number "Largest Unique Number")
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[Next >](https://github.com/openset/leetcode/tree/master/problems/connecting-cities-with-minimum-cost "Connecting Cities With Minimum Cost")
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## 1134. Armstrong Number (Easy)
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### Related Topics
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[[Math](https://github.com/openset/leetcode/tree/master/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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Check if the given k-digit number equals the sum of the k-th power of it's digits.
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</details>
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<details>
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<summary>Hint 2</summary>
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How to compute the sum of the k-th power of the digits of a number ? Can you divide the number into digits using division and modulus operations ?
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</details>
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<details>
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<summary>Hint 3</summary>
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You can find the least significant digit of a number by taking it modulus 10. And you can remove it by dividing the number by 10 (integer division). Once you have a digit, you can raise it to the power of k and add it to the sum.
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</details>

‎problems/connecting-cities-with-minimum-cost/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](https://github.com/openset/leetcode/tree/master/problems/armstrong-number "Armstrong Number")
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[Next >](https://github.com/openset/leetcode/tree/master/problems/parallel-courses "Parallel Courses")
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## 1135. Connecting Cities With Minimum Cost (Medium)
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<p>There are <code>N</code> cities numbered from 1 to <code>N</code>.</p>
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<p>You are given <code>connections</code>, where each <code>connections[i] = [city1, city2, cost]</code>&nbsp;represents the cost to connect <code>city1</code> and <code>city2</code> together.&nbsp; (A <em>connection</em> is bidirectional: connecting <code>city1</code> and <code>city2</code> is the same as connecting <code>city2</code> and <code>city1</code>.)</p>
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<p>Return the minimum cost so that for every pair of cities, there exists a path&nbsp;of connections (possibly of length 1) that connects those two cities together.&nbsp; The cost is the sum of the connection costs used. If the task is impossible, return -1.</p>
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<p>&nbsp;</p>
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<p><strong>Example 1:</strong></p>
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<p><img alt="" src="https://assets.leetcode.com/uploads/2019/04/20/1314_ex2.png" style="width: 161px; height: 141px;" /></p>
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<pre>
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<strong>Input: </strong>N = 3, connections = [[1,2,5],[1,3,6],[2,3,1]]
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<strong>Output: </strong>6
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<strong>Explanation: </strong>
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Choosing any 2 edges will connect all cities so we choose the minimum 2.
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</pre>
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<p><strong>Example 2:</strong></p>
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<p><img alt="" src="https://assets.leetcode.com/uploads/2019/04/20/1314_ex1.png" style="width: 136px; height: 91px;" /></p>
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<pre>
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<strong>Input: </strong>N = 4, connections = [[1,2,3],[3,4,4]]
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<strong>Output: </strong>-1
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<strong>Explanation: </strong>
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There is no way to connect all cities even if all edges are used.
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</pre>
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<p>&nbsp;</p>
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<p><strong>Note:</strong></p>
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<ol>
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<li><code>1 &lt;= N &lt;= 10000</code></li>
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<li><code>1 &lt;= connections.length &lt;= 10000</code></li>
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<li><code>1 &lt;= connections[i][0], connections[i][1] &lt;= N</code></li>
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<li><code>0 &lt;= connections[i][2] &lt;= 10^5</code></li>
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<li><code>connections[i][0] != connections[i][1]</code></li>
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</ol>
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### Related Topics
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[[Union Find](https://github.com/openset/leetcode/tree/master/tag/union-find/README.md)]
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[[Graph](https://github.com/openset/leetcode/tree/master/tag/graph/README.md)]
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### Hints
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<details>
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<summary>Hint 1</summary>
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What if we model the cities as a graph?
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</details>
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<details>
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<summary>Hint 2</summary>
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Build a graph of cities and find the minimum spanning tree.
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</details>
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<details>
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<summary>Hint 3</summary>
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You can use a variation of the Kruskal's algorithm for that.
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</details>
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<details>
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<summary>Hint 4</summary>
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Sort the edges by their cost and use a union-find data structure.
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</details>
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<details>
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<summary>Hint 5</summary>
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How to check all cities are connected?
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</details>
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<details>
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<summary>Hint 6</summary>
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At the beginning we have n connected components, each time we connect two components the number of connected components is reduced by one. At the end we should end with only a single component otherwise return -1.
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</details>

‎problems/largest-unique-number/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](https://github.com/openset/leetcode/tree/master/problems/reported-posts-ii "Reported Posts II")
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[Next >](https://github.com/openset/leetcode/tree/master/problems/armstrong-number "Armstrong Number")
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## 1133. Largest Unique Number (Easy)
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### Related Topics
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[[Array](https://github.com/openset/leetcode/tree/master/tag/array/README.md)]
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[[Hash Table](https://github.com/openset/leetcode/tree/master/tag/hash-table/README.md)]
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### Hints
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<details>
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<summary>Hint 1</summary>
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Find the number of occurrences of each value.
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</details>
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<details>
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<summary>Hint 2</summary>
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Use an array or a hash table to do that.
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</details>
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<details>
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<summary>Hint 3</summary>
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Look for the largest value with number of occurrences = 1.
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</details>

‎problems/parallel-courses/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](https://github.com/openset/leetcode/tree/master/problems/connecting-cities-with-minimum-cost "Connecting Cities With Minimum Cost")
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Next >
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## 1136. Parallel Courses (Hard)
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### Related Topics
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[[Dynamic Programming](https://github.com/openset/leetcode/tree/master/tag/dynamic-programming/README.md)]
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[[Depth-first Search](https://github.com/openset/leetcode/tree/master/tag/depth-first-search/README.md)]
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[[Graph](https://github.com/openset/leetcode/tree/master/tag/graph/README.md)]
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### Hints
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<details>
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<summary>Hint 1</summary>
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Try to think of it as a graph problem.
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</details>
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<details>
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<summary>Hint 2</summary>
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When will be impossible to study all the courses?
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</details>
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<details>
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<summary>Hint 3</summary>
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What if the directed graph has a cycle?
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</details>
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<details>
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<summary>Hint 4</summary>
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If you build a graph using the relations, what would be the graph type?
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</details>
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<details>
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<summary>Hint 5</summary>
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So the graph is a directed acyclic graph (DAG).
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</details>
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<details>
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<summary>Hint 6</summary>
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Imagine having a long path in the DAG, what can you say about the answer then?
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</details>
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<details>
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<summary>Hint 7</summary>
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How to find the longest path in a DAG?
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</details>
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<details>
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<summary>Hint 8</summary>
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We can use DP in order to solve this.
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</details>

‎problems/reported-posts-ii/README.md‎

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[< Previous](https://github.com/openset/leetcode/tree/master/problems/maximum-of-absolute-value-expression "Maximum of Absolute Value Expression")
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[Next >](https://github.com/openset/leetcode/tree/master/problems/largest-unique-number "Largest Unique Number")
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## 1132. Reported Posts II (Medium)
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‎tag/graph/README.md‎

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| # | 题名 | 标签 | 难度 |
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| 1135 | [最低成本联通所有城市](https://github.com/openset/leetcode/tree/master/problems/connecting-cities-with-minimum-cost) | [[并查集](https://github.com/openset/leetcode/tree/master/tag/union-find/README.md)] [[图](https://github.com/openset/leetcode/tree/master/tag/graph/README.md)] | Medium |
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| 1102 | [得分最高的路径](https://github.com/openset/leetcode/tree/master/problems/path-with-maximum-minimum-value) 🔒 | [[深度优先搜索](https://github.com/openset/leetcode/tree/master/tag/depth-first-search/README.md)] [[并查集](https://github.com/openset/leetcode/tree/master/tag/union-find/README.md)] [[图](https://github.com/openset/leetcode/tree/master/tag/graph/README.md)] | Medium |
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| 1059 | [从始点到终点的所有路径](https://github.com/openset/leetcode/tree/master/problems/all-paths-from-source-lead-to-destination) 🔒 | [[深度优先搜索](https://github.com/openset/leetcode/tree/master/tag/depth-first-search/README.md)] [[图](https://github.com/openset/leetcode/tree/master/tag/graph/README.md)] | Medium |
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| 1043 | [分隔数组以得到最大和](https://github.com/openset/leetcode/tree/master/problems/partition-array-for-maximum-sum) | [[图](https://github.com/openset/leetcode/tree/master/tag/graph/README.md)] | Medium |

‎tag/union-find/README.md‎

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| 1135 | [最低成本联通所有城市](https://github.com/openset/leetcode/tree/master/problems/connecting-cities-with-minimum-cost) | [[并查集](https://github.com/openset/leetcode/tree/master/tag/union-find/README.md)] [[图](https://github.com/openset/leetcode/tree/master/tag/graph/README.md)] | Medium |
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| 1102 | [得分最高的路径](https://github.com/openset/leetcode/tree/master/problems/path-with-maximum-minimum-value) 🔒 | [[深度优先搜索](https://github.com/openset/leetcode/tree/master/tag/depth-first-search/README.md)] [[并查集](https://github.com/openset/leetcode/tree/master/tag/union-find/README.md)] [[图](https://github.com/openset/leetcode/tree/master/tag/graph/README.md)] | Medium |
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| 1101 | [彼此熟识的最早时间](https://github.com/openset/leetcode/tree/master/problems/the-earliest-moment-when-everyone-become-friends) 🔒 | [[并查集](https://github.com/openset/leetcode/tree/master/tag/union-find/README.md)] | Medium |
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| 1061 | [按字典序排列最小的等效字符串](https://github.com/openset/leetcode/tree/master/problems/lexicographically-smallest-equivalent-string) 🔒 | [[深度优先搜索](https://github.com/openset/leetcode/tree/master/tag/depth-first-search/README.md)] [[并查集](https://github.com/openset/leetcode/tree/master/tag/union-find/README.md)] | Medium |

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