5. 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.

Example 1:

Input: "babad"
Output: "bab"
Note: "aba" is also a valid answer.

Example 2:

Input: "cbbd"
Output: "bb"

class Solution {
      private String res = "";//res全局变量
      public String longestPalindrome(String s) {
        if (s.length() <= 1) return s;
        for (int i = 0; i < s.length(); i++){
        midExpansion(s, i, i);  //for普通（aba）
        midExpansion(s, i , i + 1);   //for至少连续两个（aa）
        
        } 
           return res;
    }
        public void midExpansion(String s, int left, int right){
            while (right < s.length() && left >= 0  && s.charAt(left) == s.charAt(right)){
                left--;
                right++;
            }
         if (right - left > res.length()){
             res = s.substring(left+1, right);  //比之前的长就更新，最后一次更新完left比预期小1
         }
            
        }
}