## Two Sum

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.

You can return the answer in any order.

Here are some sample inputs and outputs:

```
Input: nums = [2,7,11,15], target = 9
Output: [0,1]
Output: Because nums[0] + nums[1] == 9, we return [0, 1].
```

```
Input: nums = [3,2,4], target = 6
Output: [1,2]
```

```
Input: nums = [3,3], target = 6
Output: [0,1]
```

## Solution

## Helpers