测试点有问题

#include<bits/stdc++.h>
using namespace std;
string a;
int s=1;
int i;
int main(){
    cin>>a;
    int len=a.size();
    while(a[i]==a[len-1-i]&&i<=len/2){
        i++;
        s=0;
    }
    if(s==0) cout<<"yes";
    else cout<<"no";
    return 0;
}

很明显错的但是能AC