1. Two Sum

Question

https://leetcode.com/problems/two-sum/

Given an array of integers, return indices of the two numbers such that they add up to a specific target.

You may assume that each input would have exactly one solution.

Example:

Given nums = [2, 7, 11, 15], target = 9,

Because nums[0] + nums[1] = 2 + 7 = 9,
return [0, 1].

Solution

Map the value to its index as we loop, so that later we can check the index of the reduced value with O(1).

/**
 * @param {number[]} nums
 * @param {number} target
 * @return {number[]}
 */
var twoSum = function(nums, target) {
    let indices = {}
    for (let i = 0; i < nums.length; i += 1) {
        if (typeof indices[target - nums[i]] !== 'undefined') {
            return [indices[target - nums[i]], i]
        }
        indices[nums[i]] = i
    }
};