1. Expand outward
Grow the palindrome while the mirrored characters match.
while (left >= 0 && right < s.length && s[left] === s[right])Typorithm · medium
Longest Palindrome
▷ Interactive algorithm lablongest-palindromestring-algorithmsexpand-centerodd-evensubstring
Saved on this device
Full lesson · 8 learning steps Open any exercise for focused practice
10%
Step 1 of 8 · intro card
Learn Longest Palindrome
String Algorithms
Expand around every center (odd and even) and keep the longest mirrored span you find.
Every palindrome has a center: a single character (odd) or a gap between two (even).
From each center, expand outward while the characters mirror, recording the longest span.
Trying both center types at each index covers all palindromes in O(n^2).
Preparing algorithm lab…
01
Centers are odd (i) and even (i, i+1).
02
Expand while mirrored chars match.
03
Track the longest span.
function longestPalindrome(s) {
let start = 0;
let maxLen = 0;
function expand(left, right) {
while (left >= 0 && right < s.length && s[left] === s[right]) {
left -= 1;
right += 1;
}
return right - left - 1;
}
for (let i = 0; i < s.length; i += 1) {
const len = Math.max(expand(i, i), expand(i, i + 1));
if (len > maxLen) {
maxLen = len;
start = i - Math.floor((len - 1) / 2);
}
}
return s.slice(start, start + maxLen);
}Read the implementation
Connect each part of the code to the concept before you type.
2. Measured length
After the loop overshoots by one on each side, the span is right - left - 1.
return right - left - 13. Odd and even
Try an odd center at i and an even center between i and i+1.
const len = Math.max(expand(i, i), expand(i, i + 1))4. Recover start
Derive the substring's start index from the center and length.
start = i - Math.floor((len - 1) / 2)A complete learning loop
01
Understand
Read the concept map and follow the code breakdown.
02
Reconstruct
Type the implementation, repair mistakes and work through every recall exercise.
03
Verify
Use the final check without hints; revisit weak concepts in short practice sessions.