leetcode上有一道类似的

https://www.acwing.com/activity/content/code/content/339912/

#include<iostream>
using namespace std;
string longestPalindrome(string s) {
        string res;
        for (int i = 0; i < s.size(); i ++ ) {
            int l = i - 1, r = i + 1;
            while (l >= 0 && r < s.size() && s[l] == s[r]) l --, r ++ ;
            if (res.size() < r - l - 1) res = s.substr(l + 1, r - l - 1);

            l = i, r = i + 1;
            while (l >= 0 && r < s.size() && s[l] == s[r]) l --, r ++ ;
            if (res.size() < r - l - 1) res = s.substr(l + 1, r - l - 1);
        }
        return res;
}
int main(){
    string s;
    getline(cin,s);
    cout<<longestPalindrome(s).size()<<endl;
    return 0;
}