Practice Two Sum

Given an array of integers nums and an integer target, return indices of the two numbers such that they add up to target. You may assume that each input would have exactly one solution, and you may not use the same element twice. You can return the answer in any order. Practice approach: Scan left to right while mapping each previously seen value to an index. Before storing the current value, look for target minus that value. This order prevents using the current index twice and still handles equal values: for [3, 3] and target 6, the first 3 is stored and the second finds it. Expected time is O(n) with a hash map and O(n) additional space. Return indices rather than values, and test both a repeated-value pair and a pair separated by unrelated entries.