chore(JavaScript): add two sum (#1031)

Co-authored-by: Ming Tsai <37890026+ming-tsai@users.noreply.github.com>

algorithms/JavaScript/README.md

@@ -3,6 +3,7 @@
 ## Arrays
 
 - [Counting Inversions](src/arrays/counting-inversions.js)
+- [Two Sum](src/arrays/two-sum.js)
 - [Single Occurring Element](src/arrays/single-occurring-element.js)
 
 ## Linked Lists

algorithms/JavaScript/src/arrays/two-sum.js

@@ -0,0 +1,52 @@
+// Documentation credit: Himanshu from two-sum.go
+/*
+    Problem Statement : Given an array of integers nums and an integer target,
+    return indices of the two numbers such that they add up to target.
+    You may assume that each input would have exactly one solution,
+    and you may not use the same element twice.
+
+    Input: An array of integers and a target (int)
+    Output: array of indexes of len(2) with sum of element
+    at that index equal to target or nil
+*/
+
+
+/*
+Using Brute Force : For every element check for another element if
+    it exist in the array such that sum of both the element is equals
+    to the target
+    Time Complexity   : O(n^2)
+*/
+const twoSumBrute = (arr, target) => {
+  for (let i = 0; i < arr.length; i++) {
+    for (let j = 0; j < arr.length; j++) {
+      if (arr[i] + arr[j] === target) {
+        return [i, j];
+      }
+    }
+  }
+  return [];
+};
+
+/*
+    Using Map := While traversing every element add the element as key
+    and its position as its value in a map Check the required value
+    (i.e target - arr[i]) in the map If the map contains the required value
+    then we have two elements with the required sum and return the positions.
+    Time Complexity : O(n)
+*/
+
+const twoSum = (arr, target) => {
+  const map = new Map();
+
+  for (let i = 0; i < arr.length; i++) {
+    if (map.has(arr[i])) return [map.get(arr[i]), i];
+    map.set(target - arr[i], i);
+  }
+  return [];
+};
+
+const nums = [2, 7, 11, 15];
+const target = 9;
+console.log(`Two Sum brute: ${twoSumBrute(nums, target)}`);
+console.log(`Two sum optimized: ${twoSum(nums, target)}`);