Given an array of integers, return the indices of the two numbers such that they add up to a specific target. You may assume that each input would have exactly one solution, and you may not use the same element twice. Example 1: Input: nums = [2,7,11,15], target = 9 Output: [0,1] Example 2: Input: nums = [3,2,4], target = 6 Output: [1,2] Constraints: - 2 <= nums.length <= 10^4 - -10^9 <= nums[i] <= 10^9 - -10^9 <= target <= 10^9 - Only one valid answer exists, and the same element cannot be used twice.