Two Sum II (Sorted Input)
Module 10 · Two Pointers
Problem
Given a 1-indexed, sorted array and a target, return the indices of the two numbers summing to target. Exactly one solution exists. Required: O(1) extra space (which rules out Module 6's hash map).
Examples
Example 1
Input
numbers = [2,7,11,15], target = 9Output[1,2]Example 2
Input
numbers = [2,3,4], target = 6Output[1,3]Example 3
Input
numbers = [-1,0], target = -1Output[1,2]Constraints
2 ≤ n ≤ 3·10⁴ · sorted non-decreasing · exactly one answer.
Attempt it first
The converging lesson used this exact problem to build the elimination argument — so the test here is whether you can reproduce the proof, not just the loop. Write the solution AND, in a comment, the one-sentence reason each pointer move is safe. Then compare.
Indices are 1-based, as the problem states — a common way to get this right in logic and wrong in output.