## 1. Two Sum *| Difficulty | Easy |*

*| Difficulty | Easy |*

## Problem:

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, and you may not use the *same* element twice.

## Example:

**Example:**

```
Given nums = [2, 7, 11, 15], target = 9,
Because nums[0] + nums[1] = 2 + 7 = 9,
return [0, 1].
```

## Solution:

Map the numbers to their indices as we iterate, so that we can look them back up in O(1) time complexity.

```
/**
* @param {number[]} nums
* @param {number} target
* @return {number[]}
*/
var twoSum = function (nums, target) {
const indicies = {};
for (let i = 0; i < nums.length; i++) {
const num = nums[i];
if (indicies[target - num] != null) {
return [indicies[target - num], i];
}
indicies[num] = i;
}
};
// Ans:
// | Your answer | Expected answer |
// | [0,1] | [0,1] |
```