Longest Palindromic Substring

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.

给定一个字符串，求出其最长回文子串。

 

public class Solution {
    public String longestPalindrome(String s) {
        if (s.length() == 0) {
        	return null;
        }
        if (s.length() == 1) {
        	return s;
        }
        String longest = s.substring(0, 1);
        for (int i = 0; i < s.length(); i++) {
        	String t1 = helper(i, i, s);
        	if (longest.length() < t1.length()) {
        		longest = t1;
        	}
        	String t2 = helper(i, i+1, s);
        	if (longest.length() < t2.length()) {
        		longest = t2;
        	}
        }
        return longest;
    }

	private String helper(int beg, int end, String s) {
		while (beg>-1 && end<s.length() && s.charAt(beg)==s.charAt(end)) {
			beg--;
			end++;
		}
		return s.substring(beg+1, end);
	}
    
}