add: twoSum solution.

Author: albin3

algorithms/js/twoSum/twoSum.js

@@ -0,0 +1,36 @@
+// Source : https://oj.leetcode.com/problems/two-sum/
+// Author : Albin Zeng.
+// Date   : 2017-02-26
+
+/********************************************************************************** 
+* 
+* Given an array of integers, find two numbers such that they add up to a specific target number.
+* 
+* The function twoSum should return indices of the two numbers such that they add up to the target, 
+* where index1 must be less than index2. Please note that your returned answers (both index1 and index2) 
+* are not zero-based.
+* 
+* You may assume that each input would have exactly one solution.
+* 
+* Input: numbers={2, 7, 11, 15}, target=9
+* Output: index1=1, index2=2
+* 
+*               
+**********************************************************************************/
+
+/**
+ * @param {number[]} nums
+ * @param {number} target
+ * @return {number[]}
+ */
+var twoSum = function(nums, target) {
+    const map = {};
+    let i = 0;
+    for (; i < nums.length; i++) {
+        if (undefined === map[target - nums[i]]) {
+            map[nums[i]] = i;
+        } else {
+            return [map[target - nums[i]], i];
+        }
+    }
+};