13. Longest Palindromic Substring

mediumAsked at Swiggy

Return the longest substring of s that reads the same forwards and backwards.

By Alex Chen, Founder, InterviewChamp.AI · Last verified 2026-05-20

Problem

Given a string s of length n, return the longest palindromic substring contained in s. If multiple have the same maximum length, return any one.

Constraints

1 <= s.length <= 1000
s contains only printable ASCII

Examples

Example 1

Input

s='babad'

Output

'bab' or 'aba'

Example 2

Input

s='cbbd'

Output

'bb'

Approaches

1. Brute substring check

Test every substring for palindrome property.

Time: O(n^3)
Space: O(1)

function isPal(t){return t===t.split('').reverse().join('');}
let best='';
for (let i=0;i<s.length;i++)
  for (let j=i+1;j<=s.length;j++)
    if (isPal(s.slice(i,j)) && j-i>best.length) best=s.slice(i,j);
return best;

Tradeoff:

2. Expand around centers

Every palindrome has a center (single char or gap between two chars). For each of the 2n-1 centers, expand outward while characters match; track the best span.

Time: O(n^2)
Space: O(1)

function longestPalindrome(s) {
  let start = 0, end = 0;
  const expand = (l, r) => {
    while (l >= 0 && r < s.length && s[l] === s[r]) { l--; r++; }
    return [l + 1, r - 1];
  };
  for (let i = 0; i < s.length; i++) {
    for (const [l, r] of [expand(i, i), expand(i, i + 1)]) {
      if (r - l > end - start) { start = l; end = r; }
    }
  }
  return s.slice(start, end + 1);
}

Tradeoff:

Swiggy-specific tips

Swiggy treats this as a 'comfort with two-pointer expansion' check that maps onto symmetric route validation in their restaurant-graph problems.

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Output

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