Two pointers is less a single algorithm than a way to avoid repeating work. The pattern becomes useful when two positions can move through ordered data while preserving a clear invariant.
Signals that suggest two pointers
Look for a sorted sequence, a pair or interval, or a requirement to compare values from opposite ends. Common prompts ask for a target sum, removal of duplicates, interval contraction, or an in-place partition. The strongest clue is that moving one pointer lets you rule out many candidates at once.
Choose the movement rule first
Suppose a sorted list must contain two values whose sum equals a target. If the current sum is too small, advancing the left pointer is the only move that can increase it. If the sum is too large, retreating the right pointer is the only move that can decrease it. State this rule before writing the loop.
Protect the invariant
A good explanation names what remains true: every discarded pair is impossible, and any valid answer still lies between the pointers. Test an empty list, one item, duplicates, negative values, and the case where no pair exists. This turns a coding trick into a defensible algorithm.
Continue with the book
Explore the full treatment, worked examples, and exercises in Algorithmic Edge.
Adapted from Algorithmic Edge, published by Choral Software. Article text and original examples © 2026 Choral Software. All rights reserved.