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.
寻找最长的回文字符串。用的是一个比较简单的方式。即每一个字符开始比较它之前和之后的字符来判断是否为相同，若相同，则继续向前，向后比较，直至到不相同。判断回文字符串的长度是否更长。
public class Solution {
    public String longestPalindrome(String s) {
String str = "";
        int midIndex = 0 ;
        int num = 0 ;
        boolean isOnly = false;
        for(int i = 0;i<s.length()-1;i++){
        if(s.length()-1 - i < num){
        break;
        }else{       
        if(s.charAt(1+i) == s.charAt(i)){
        int j = 1;
       
         while(i-j>=0&&i+j+1<s.length()&&s.charAt(i-j) == s.charAt(i+j+1)){
        j++;
        }
        if(num < j){
        num = j;
        midIndex = i;
        isOnly = false;
        }
        }
        if(i!= 0&&s.charAt(1+i) == s.charAt(i-1)){
        int j = 1 ;
        while(i-j>=0&&i+j<s.length()&&s.charAt(i+j) == s.charAt(i-j)){
       
        j++;
       
        }
        j--;
        if(num <= j){
       
        num = j;
        midIndex = i;
        isOnly = true;
        }
       
       
        }
             
        }
       
       
        }
        if(num ==0){
       
        str = s.charAt(0)+"";
        }else if(isOnly){
        str = s.substring(midIndex - num, midIndex + num+1);
        }else{
        str = s.substring(midIndex- num+1,midIndex +num +1) ;
        }
       
        return str;
    }
}