LeetCode 5: Longest Palindromic Substring in Java

Every palindrome has a center. Expand around each character for odd-length palindromes and around each adjacent pair for even-length palindromes.

class Solution {
    public String longestPalindrome(String s) {
        if (s == null || s.length() < 2) return s;

        int start = 0;
        int end = 0;

        for (int i = 0; i < s.length(); i++) {
            int odd = expand(s, i, i);
            int even = expand(s, i, i + 1);
            int len = Math.max(odd, even);

            if (len > end - start + 1) {
                start = i - (len - 1) / 2;
                end = i + len / 2;
            }
        }

        return s.substring(start, end + 1);
    }

    private int expand(String s, int left, int right) {
        while (left >= 0 && right < s.length()
                && s.charAt(left) == s.charAt(right)) {
            left--;
            right++;
        }
        return right - left - 1;
    }
}

This approach uses O(n²) time and O(1) extra space. A dynamic-programming solution also takes O(n²) time but requires O(n²) memory. Manacher's algorithm reaches linear time but is substantially more complex.