Longest Palindromic Substring最长回文子串(5 Medium)

Given a string S, find the longest palindromic substring in S. You may assume that the maximum length of S is 1000, and there exists one unique longest palindromic substring.

Time O(n^2), Space O(1),expand from center

public String longestPalindrome(String s) {
    if (s.isEmpty()) {
        return null;
    }

    if (s.length() == 1) {
        return s;
    }

    String longest = s.substring(0, 1);
    for (int i = 0; i < s.length(); i++) {
        // get longest palindrome with center of i
        String tmp = helper(s, i, i);
        if (tmp.length() > longest.length()) {
            longest = tmp;
        }

        // get longest palindrome with center of i, i+1
        tmp = helper(s, i, i + 1);
        if (tmp.length() > longest.length()) {
            longest = tmp;
        }
    }

    return longest;
}

// Given a center, either one letter or two letter, 
// Find longest palindrome
public String helper(String s, int begin, int end) {
    while (begin >= 0 && end <= s.length() - 1 && s.charAt(begin) == s.charAt(end)) {
        begin--;
        end++;
    }
    return s.substring(begin + 1, end);
}