Your question is Two Sum in Unsorted Array. Start with the requirements on the right.
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Given an unsorted integer array nums and an integer target, return the indices of the two distinct elements whose sum equals target. Exactly one valid answer exists, and you cannot use the same element twice.
2 <= len(nums) <= 10^5-10^9 <= nums[i] <= 10^9-10^9 <= target <= 10^9