# 1. Two Sum (Easy)

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.

## Example 1:

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

## Example 2:

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

## Example 3:

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

## Constraints:

- 2 <= nums.length <= 10^4
- -10^9 <= nums[i] <= 10^9
- -10^9 <= target <= 10^9
- **Only one valid answer exists.**

Follow-up: Can you come up with an algorithm that is less than `O(n^2)` time
complexity?