Two Pointers

Use two indices moving toward each other or in the same direction to solve linear problems.

13 problems 5 Easy 7 Medium 1 Hard

Overview

Two pointers eliminates the need for nested loops by having two references into the data that move based on conditions. Works on sorted arrays or arrays where a monotonic property exists.

When to Use

Pair sum in sorted array
Removing duplicates in-place
Container with most water
Three-sum and its variants
Checking palindromes

Common Mistakes

Using two pointers on unsorted data without justification
Infinite loops when neither pointer moves
Handling duplicates incorrectly in three-sum

Complexity

Time

O(n) — both pointers traverse at most n elements total

Space

O(1) — no extra space needed

function twoPointers(arr) {
  let left = 0, right = arr.length - 1;

  while (left < right) {
    const sum = arr[left] + arr[right];
    if (sum === target) {
      return [left, right];
    } else if (sum < target) {
      left++;
    } else {
      right--;
    }
  }
  return [];
}

Problems (13)

Hide completed

Problem	Difficulty	Companies	Freq
Two Sum II - Input Array Is Sorted	Medium	AmazonApple	High
3Sum	Medium	FacebookAmazon +2	High
Container With Most Water	Medium	AmazonFacebook +2	High
Trapping Rain Water	Hard	AmazonGoogle +2	High
Valid Palindrome	Easy	FacebookMicrosoft +1	High
Move Zeroes	Easy	FacebookBloomberg	Medium
Remove Duplicates from Sorted Array	Easy	AppleMicrosoft	High
4Sum	Medium	AmazonAdobe	Medium
Sort Colors	Medium	FacebookMicrosoft +1	High
Squares of a Sorted Array	Easy	GoogleAmazon +1	High
Boats to Save People	Medium	AmazonGoogle	Medium
Valid Palindrome II	Easy	FacebookAmazon +1	High
Minimum Length of String After Deleting Similar Ends	Medium	Google	Low

Related Guide

Two Pointers: The Complete Guide →

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