Given an array of integers nums and an integer target, return the indices of the two distinct elements whose values add up to target. The input is guaranteed to contain exactly one valid pair, and the same element cannot be used twice.
Input: nums = [2, 7, 11, 15], target = 9
Output: [0, 1]
Explanation: nums[0] + nums[1] = 2 + 7 = 9.
Input: nums = [3, 2, 4], target = 6
Output: [1, 2]
Explanation: nums[1] + nums[2] = 2 + 4 = 6.
2 <= len(nums) <= 10^5-10^9 <= nums[i] <= 10^9-10^9 <= target <= 10^9