## What is it? Given a sorted array of integers and a target sum, find the two indices whose values add up to the target. The classic two-pointer problem on a sorted array (LeetCode 167). ## How it works - Place `left = 0` (beginning), `right = n-1` (end) - If `arr[left] + arr[right] == target` → return `[left+1, right+1]` (1-indexed) - If sum < target → move `left++` (need a larger value) - If sum > target → move `right--` (need a smaller value) - Repeat until found ## When to use - Two sum on a sorted array - Any "find a pair summing to target" in sorted data - Foundation for Three Sum (sort first, then apply two pointers) ## Key Points - Works only on sorted arrays — sort first if needed (but then it costs O(n log n)) - O(n) time, O(1) space — optimal for sorted input - Guaranteed to find a solution by the problem constraints (exactly one solution)