LongestPalindromeSubstringM1.java

/**
 * Leetcode problem #5, Longest Palindromic Substring
 * https://leetcode.com/problems/longest-palindromic-substring/
 */
public class LongestPalindromeSubstringM1 {
    public static void main(String[] args) {
        String s = "babad";
        String palindrome = longestPalindrome(s);
        System.out.println(palindrome);
    }

    public static String longestPalindrome(String s) {
        if (s == null || s.length() < 1)
            return "";

        int start = 0, end = 0;
        for (int i = 0; i < s.length(); i++) {
            int len1 = expandAroundCenter(s, i, i);
            int len2 = expandAroundCenter(s, i, i + 1);
            int len = Math.max(len1, len2);

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

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

    public static int expandAroundCenter(String s, int l, int r) {
        while (l >= 0 && r < s.length() && s.charAt(l) == s.charAt(r)) {
            l--;
            r++;
        }

        return r - l - 1;
    }
}