[Jeehay Park(박지혜)] Week 2

3sum/Jeehay28.js

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+/**
+ * @param {number[]} nums
+ * @return {number[][]}
+ */
+
+
+// Time Complexity: O(n^2)
+// Space Complexity: O(n^2)
+
+var threeSum = function (nums) {
+
+    const sorted = [...nums].sort((a, b) => a - b);
+
+    let result = [];
+
+    // Loop through the array and pick each number as the first number for the triplet
+    for (let i = 0; i < sorted.length - 2; i++) {
+
+        // skip duplicate values for sorted[middle]
+        if(i > 0 && sorted[i - 1] === sorted[i]) {
+            continue;
+        }
+
+        let left = i + 1; // Left pointer starts right after the current middle
+        let right = sorted.length - 1; // Right pointer starts at the last element
+
+        while (left < right) {
+            const sum = sorted[i] + sorted[left] + sorted[right];
+
+            if (sum === 0) {
+                result.push([sorted[left], sorted[i], sorted[right]]);
+
+                // skip duplicates for sorted[left] and sorted[right]
+                while(left < right && sorted[left] === sorted[left + 1]){
+                    left += 1; // Move left pointer to the right to skip duplicate values
+                } 
+
+                while(left < right && sorted[right] === sorted[right - 1]) {
+                    right -= 1; // Move right pointer to the left to skip duplicate values
+                }
+
+                left += 1; 
+                right -= 1;
+
+            } else if (sum > 0) {
+                right -= 1;
+
+            } else {
+                left += 1
+            }
+        }
+    }
+
+     return result;
+
+};
+
+
+// var threeSum = function (nums) {
+
+// i != j, i != k, and j != k can be interpreted as i < j < k
+
+    // three nested loops
+    // time complexity of O(n³)
+    // Time Limit Exceeded
+
+    // let result = [];
+
+    // for (let i = 0; i < nums.length - 2; i++) {
+    //     for (let j = i + 1; j < nums.length - 1; j++) {
+    //         for (let k = j + 1; k < nums.length; k++) {
+    //             if (nums[i] + nums[j] + nums[k] === 0) {
+    //                 const str = [nums[i], nums[j], nums[k]].sort((a, b) => a - b).join(",")
+    //                 result.push(str)
+    //             }
+    //         }
+    //     }
+
+    // }
+
+    // result = [... new Set(result)].map(str => str.split(",").map(str => +str))
+
+    // return result;
+
+// }
+

climbing-stairs/Jeehay28.js

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+/**
+ * @param {number} n
+ * @return {number}
+ */
+
+// Time Complexity: O(n^2)
+// First O(n): From the loop that runs up to n times.
+// Second O(n): From the factorial calculations in each iteration.
+// overall time complexity is O(n^2).
+
+// Space Complexity: O(n)
+// the factorial function's recursion has a space complexity of O(n)
+// O(1) means constant space complexity. It implies that the amount of memory used does not grow with the size of the input.
+// The other variables in the function only use a constant amount of space, resulting in an O(1) space usage.
+
+var climbStairs = function (n) {
+  let ways = 0;
+
+  let maxStepTwo = Math.floor(n / 2);
+
+  const factorial = (num) => {
+    if (num === 0 || num === 1) {
+      return 1;
+    }
+
+    for (let i = 2; i <= num; i++) {
+      return num * factorial(num - 1);
+    }
+  };
+
+  for (let i = 0; i <= maxStepTwo; i++) {
+    const stepTwo = i;
+    const stepOne = n - 2 * i;
+    const steps = stepTwo + stepOne;
+
+    ways += factorial(steps) / (factorial(stepTwo) * factorial(stepOne));
+  }
+
+  return ways;
+};
+

valid-anagram/Jeehay28.js

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+/**
+ * @param {string} s
+ * @param {string} t
+ * @return {boolean}
+ */
+
+// 시간 복잡도: O(n)
+// 공간 복잡도: O(n) 
+
+var isAnagram = function (s, t) {
+
+    if (s.length !== t.length) {
+        return false;
+    }
+
+    let obj = {};
+
+    for (let k of s) {
+        obj[k] = (obj[k] || 0) + 1;
+
+    }
+
+    for (let k of t) {
+        if (obj[k] === undefined || obj[k] === 0) {
+            return false;
+        }
+        obj[k]--;
+    }
+
+    return true;
+
+};
+
+// 시간 복잡도: O(n log n)
+// 공간 복잡도: O(n)
+
+// var isAnagram = function (s, t) {
+
+//     if (s.length !== t.length) {
+//         return false;
+//     }
+
+//     let sArr = s.split("").sort();
+//     let tArr = t.split("").sort();
+
+//     for (let i = 0; i < sArr.length; i++) {
+//         if (sArr[i] !== tArr[i]) {
+//             return false;
+//         }
+//     }
+
+//     return true;
+
+// };
+