回文串最大长度

1）lettcode题目：

Given a string s, find the longest palindromic substring in s. You may assume that the maximum length of s is 1000.

Example:

Input: "babad"

Output: "bab"

Note: "aba" is also a valid answer.

class Solution {
public:
//以每个字符为中心，向左右拓展，找最大回文串
    string expandAroundCenter(string s , int c1, int c2){
        int l=c1, r=c2;
        int n=s.length();
        while(l>=0&&r<n&&s[l]==s[r]){
            l--;
            r++;
        }
        return s.substr(l+1,r-l-1);
    }
    
    string longestPalindrome(string s) {
        int n=s.length();
        if(n==0) return "";
        string longest=s.substr(0,1);
        for(int i=0;i<n-1;i++){
            string p1=expandAroundCenter(s,i,i);
            if(p1.length()>longest.length())
                longest=p1;
            string p2=expandAroundCenter(s,i,i+1);
            if(p2.length()>longest.length())
                longest=p2;
        }
        return longest;
    }
};