https://ac.nowcoder.com/acm/problem/214851

#include <iostream>
#include <string>
using namespace std;
string s;
int n;
int fun()
{
    // 1. 判断是否全都是相同字符
    bool flag = false;
    for (int i = 1; i < n; ++i) 
    {
        if (s[i] != s[0])
        {
            flag = true;
            break;
        }
    }
    if (flag == false) return 0;
    
    // 2. 判断本身是否是回文
    flag = true;
    int left = 0, right = n-1;
    while (left < right) 
    {
        if (s[left] == s[right])
        {
            left++;
            right--;
        }
        else 
        {
            flag = false;
            break;
        }
    }
    
    if (flag) return n-1;
    else return n;
}

int main() 
{
    cin >> s;
    n = s.size();
    cout << fun() << endl;  
    return 0;
}