Common String Problems

Written by Swapnil RajaPublished 28 Aug 2026

Four patterns cover most string questions. Once you can see which one you are looking at, the code is short.

Reversing

Two ways to reverse

javascript

// Readable. O(n) time, O(n) space.
function reverse(s) {
  return [...s].reverse().join("")
}

// In place on an array of characters. O(1) extra space.
function reverseChars(chars) {
  let left = 0
  let right = chars.length - 1

  while (left < right) {
    const temp = chars[left]
    chars[left] = chars[right]
    chars[right] = temp
    left++
    right--
  }

  return chars
}

PreviousStrings in JavaScript for DSA NextHash Maps and Sets in JavaScript

Palindrome, cleaned

javascript

function isPalindrome(s) {
  const clean = s.toLowerCase().replace(/[^a-z0-9]/g, "")

  let left = 0
  let right = clean.length - 1

  while (left < right) {
    if (clean[left] !== clean[right]) return false
    left++
    right--
  }

  return true
}

console.log(isPalindrome("A man, a plan, a canal: Panama"))  // true

Two passes beat nested loops

javascript

function firstUnique(s) {
  const counts = new Map()
  for (const ch of s) counts.set(ch, (counts.get(ch) || 0) + 1)

  for (let i = 0; i < s.length; i++) {
    if (counts.get(s[i]) === 1) return i
  }

  return -1
}

console.log(firstUnique("leetcode"))  // 0
console.log(firstUnique("aabb"))      // -1

Sliding window

javascript

function longestUnique(s) {
  const lastSeen = new Map()
  let best = 0
  let start = 0

  for (let i = 0; i < s.length; i++) {
    const ch = s[i]

    if (lastSeen.has(ch) && lastSeen.get(ch) >= start) {
      start = lastSeen.get(ch) + 1
    }

    lastSeen.set(ch, i)
    best = Math.max(best, i - start + 1)
  }

  return best
}

console.log(longestUnique("abcabcbb"))  // 3

Common String Problems

Reversing

Palindromes

First non-repeating character

Longest run of unique characters