Find the Two Indices Whose Values Sum to a Target

Given an integer array and a target, return the indices of two distinct elements whose sum equals the target. ### Constraints and Clarifications The report names Two Sum without additional variant details. Use the [official reference](https://leetcode.com/problems/two-sum/description/) for this practice contract: 2 <= n <= 10000, values and target between -1000000000 and 1000000000, and exactly one valid unordered index pair. These are reference-context assumptions, not separately reported interview constraints. Return the two zero-based indices in increasing order as an output-normalization convention. Do not use the same element twice. ### Example For [4,1,6] and target 10, return [0,2]. For [5,5] and target 10, return [0,1]. ### What a Strong Answer Covers Improve on pairwise enumeration with a lookup map, explain why lookup comes before insertion, preserve original indices, and account for duplicate values. ### Follow-up Questions - How would the contract change if there were no solution or several valid pairs? - What tradeoff would a sorting-based approach introduce?