Find the Longest Palindromic Substring

mediumcodingAlgorithmsStringsDynamic Programming~30 min

Asked at Google, Meta, Shopify

Overview

Given a string s, return the longest palindromic substring in s.

If there are multiple answers of the same length, return the first one found.

Constraints

1 <= s.length <= 1000
s consists of only lowercase English letters.
O(n²) expand-around-center is the expected approach.

Examples

longestPalindrome("babad");
// => "bab" (or "aba" — both valid)

longestPalindrome("cbbd");
// => "bb"

longestPalindrome("a");
// => "a"

Solution

Reveal solution

function longestPalindrome(s) {
  let start = 0, maxLen = 1;

  function expand(l, r) {
    while (l >= 0 && r < s.length && s[l] === s[r]) {
      if (r - l + 1 > maxLen) {
        start = l;
        maxLen = r - l + 1;
      }
      l--;
      r++;
    }
  }

  for (let i = 0; i < s.length; i++) {
    expand(i, i);     // odd-length
    expand(i, i + 1); // even-length
  }

  return s.substring(start, start + maxLen);
}

Expand around every index as center for both odd and even length palindromes. O(n²) time, O(1) space.

Resources

longest-palindromic-substring.js

Find the Longest Palindromic Substring

mediumcodingAlgorithmsStrings

Overview

Given a string s, return the longest palindromic substring in s.

If there are multiple answers of the same length, return the first one found.

Constraints

1 <= s.length <= 1000
s consists of only lowercase English letters.
O(n²) expand-around-center is the expected approach.

Examples

longestPalindrome("babad");
// => "bab" (or "aba" — both valid)

longestPalindrome("cbbd");
// => "bb"

longestPalindrome("a");
// => "a"

Solution

Reveal solution

function longestPalindrome(s) {
  let start = 0, maxLen = 1;

  function expand(l, r) {
    while (l >= 0 && r < s.length && s[l] === s[r]) {
      if (r - l + 1 > maxLen) {
        start = l;
        maxLen = r - l + 1;
      }
      l--;
      r++;
    }
  }

  for (let i = 0; i < s.length; i++) {
    expand(i, i);     // odd-length
    expand(i, i + 1); // even-length
  }

  return s.substring(start, start + maxLen);
}

Expand around every index as center for both odd and even length palindromes. O(n²) time, O(1) space.

Resources

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