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 <= 104`
- `-109 <= nums[i] <= 109`
- `-109 <= target <= 109`
- Only one valid answer exists.

Follow-up: Can you come up with an algorithm that is less than `O(n2)` time complexity?