Longest Palindromic Substring

public class Solution {
    public String longestPalindrome(String s) {
        if(s==null) return null;
        if(s.length()==1) return s;
        
        String longest = s.substring(0,1); // because at least there are 2 elements in s
        
        for(int i=0;i<s.length();i++){
            String tmp = helper(s, i, i); // 中心对称外延，延伸出来的tmp为奇数位长度
            if(longest.length()<tmp.length())
                longest = tmp;
            
             tmp = helper(s,i, i+1); //中心对称外延，tmp为偶数长度
            if(longest.length()<tmp.length())
                longest = tmp;
        }
        
        return longest;
        
    
    }
    
    public String helper(String s, int b, int e){
        while(b>=0 && e< s.length()  && s.charAt(b)==s.charAt(e)){
            b--;
            e++;
        }
        return s.substring(b+1, e);
    }
}