Merge pull request #98 from nhistory/week5

[Sehwan]Week5 solutions with javascript

Author: DaleSeo

3sum/nhistory.js

@@ -0,0 +1,43 @@
+var threeSum = function (nums) {
+  // Sort nums array
+  const sortedNums = nums.sort((a, b) => a - b);
+  let result = [];
+
+  // Start iteration to pick first element for 3sum
+  for (let i = 0; i < sortedNums.length; i++) {
+    // Check if the first element is already greater than 0, no valid triplets possible after this point
+    if (sortedNums[i] > 0) {
+      break;
+    }
+
+    // Skip duplicates of the first element to avoid redundant triplets
+    if (i > 0 && sortedNums[i] === sortedNums[i - 1]) {
+      continue;
+    }
+
+    // Iterate to find sum of two pointer and nums[i]
+    let left = i + 1;
+    let right = sortedNums.length - 1;
+
+    while (left < right) {
+      let sum = sortedNums[i] + sortedNums[left] + sortedNums[right];
+
+      if (sum === 0) {
+        result.push([sortedNums[i], sortedNums[left], sortedNums[right]]);
+        // Skip duplicates of left and right pointers to avoid redundant triplets
+        while (sortedNums[left] === sortedNums[left + 1]) left++;
+        while (sortedNums[right] === sortedNums[right - 1]) right--;
+        left++;
+        right--;
+      } else if (sum < 0) {
+        left++;
+      } else if (sum > 0) {
+        right--;
+      }
+    }
+  }
+  return result;
+};
+
+// TC: O(n^2)
+// SC: O(n)

encode-and-decode-strings/nhistory.js

@@ -0,0 +1,43 @@
+class Solution {
+  /**
+   * @param {string[]} strs
+   * @returns {string}
+   */
+  encode(strs) {
+    let result = "";
+    for (let str of strs) {
+      result += str.length.toString() + "#" + str;
+    }
+    return result;
+  }
+
+  /**
+   * @param {string} str
+   * @returns {string[]}
+   */
+  decode(str) {
+    let result = [];
+    let i = 0;
+
+    while (i < str.length) {
+      // Find the position of the next '#'
+      let j = i;
+      while (str[j] !== "#") {
+        j++;
+      }
+
+      // Length of the next string
+      const len = parseInt(str.slice(i, j));
+      i = j + 1; // Move past the '#'
+
+      // Extract the string of length 'len'
+      result.push(str.slice(i, i + len));
+      i += len; // Move past the extracted string
+    }
+
+    return result;
+  }
+}
+
+// TC: O(n),O(n)
+// SC: O(n),O(n)

longest-consecutive-sequence/nhistory.js

@@ -0,0 +1,30 @@
+var longestConsecutive = function (nums) {
+  // Return 0 if there are no elements in nums
+  if (nums.length === 0) return 0;
+
+  // Create a set to find values efficiently
+  const numSet = new Set(nums);
+  let maxLength = 0;
+
+  for (let num of numSet) {
+    // Check if this is the start of a sequence
+    if (!numSet.has(num - 1)) {
+      let currentNum = num;
+      let currentLength = 1;
+
+      // Count the length of the sequence
+      while (numSet.has(currentNum + 1)) {
+        currentNum += 1;
+        currentLength += 1;
+      }
+
+      // Update the maximum length
+      maxLength = Math.max(maxLength, currentLength);
+    }
+  }
+
+  return maxLength;
+};
+
+// TC: O(n)
+// SC: O(n)

product-of-array-except-self/nhistory.js

@@ -0,0 +1,40 @@
+/**
+ * @param {number[]} nums
+ * @return {number[]}
+ */
+var productExceptSelf = function (nums) {
+  const len = nums.length;
+  const lastIndex = len - 1;
+
+  // Declare prefix, postfix array
+  let prefix = new Array(len).fill(1);
+  let postfix = new Array(len).fill(1);
+
+  // Iterate loop to add prefix[i-1]*nums[i] values into prefix array
+  prefix[0] = nums[0];
+  for (let i = 1; i < nums.length; i++) {
+    prefix[i] = prefix[i - 1] * nums[i];
+  }
+
+  // Iterate loop to add postfix[i]*nums[i-1] values into postfix array
+  postfix[lastIndex] = nums[lastIndex];
+  for (let i = lastIndex; i >= 1; i--) {
+    postfix[i - 1] = postfix[i] * nums[i - 1];
+  }
+
+  // Make output array with prefix and postfix arrays
+  let output = new Array(len).fill(1);
+
+  // First index value is equal to postfix[1]
+  output[0] = postfix[1];
+  // Last index value is equal to prefix[lastIndex-1]
+  output[lastIndex] = prefix[lastIndex - 1];
+  for (let i = 1; i < len - 1; i++) {
+    output[i] = prefix[i - 1] * postfix[i + 1];
+  }
+
+  return output;
+};
+
+// TC: O(n)
+// SC: O(n)

top-k-frequent-elements/nhistory.js

@@ -0,0 +1,14 @@
+var topKFrequent = function (nums, k) {
+  // 1. Hashmap { num : frequency }
+  let map = {};
+
+  // 2. Iterate counts frequency of each value
+  for (num of nums) {
+    // Check there is already num key or not
+    if (!map[num]) map[num] = 0;
+    map[num]++;
+  }
+
+  // 3. Sort frequency and return sliced array
+  return [...Object.keys(map)].sort((a, b) => map[b] - map[a]).slice(0, k);
+};