# 1. Two Sum

#### Question

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:**

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

#### One loop

- loop through the array
- store each number’s complement number
`target - nums[i]`

in a new array - check the complement array to see if the current number exist

#### Playground

See the Pen 1. Two Sum by Cherry Wang (@chryw) on CodePen.