Two Sum

Given an array of integers nums and an integer target, return the indices of the two numbers that add up to target.

• You may assume exactly one solution exists.
• You may not use the same element twice.
• Return the answer in any order.

Example:

Input:  nums = [2, 7, 11, 15], target = 9
Output: [0, 1]  (because nums[0] + nums[1] = 2 + 7 = 9)

Constraints:

• 2 ≤ nums.length ≤ 10⁴
• -10⁹ ≤ nums[i] ≤ 10⁹