Two Sum
Module 6 · Hash Tables
Problem
Given nums and target, return the indices of the two numbers that
sum to target. Exactly one solution exists; you may not use the same
element twice.
Goal
Return a pair of indices — not the values. Order of the pair does not matter to the judge as long as both indices are correct.
Examples
Example 1
Input
nums = [2,7,11,15], target = 9Output[0,1]Example 2
Input
nums = [3,2,4], target = 6Output[1,2]Explanation. not [0,0]!
Example 3
Input
nums = [3,3], target = 6Output[0,1]Constraints
2 ≤ n ≤ 10⁴ · values in ±10⁹ · exactly one answer
Attempt it first
Tip
You've known the O(n) decision version ("does a pair exist?") since lesson one of Big O. This asks for indices — which changes the structure you carry. Watch
[3,2,4]and[3,3]: both trap specific wrong versions.