I have WA in this test.
It's my code:

#include <iostream>
#include <string>
#include <algorithm>
#define N 1111
using namespace std;

string s;
int n,d1[N],d2[N],k;

int main(){
    getline(cin,s);
    n=s.length();
    for (int i=0;i<n;i++){
        for (k=1;i-k>=0 && i+k<n;k++) if (s[i-k]!=s[i+k]) break;
        d1[i]=max(k-1,-1);
    }
    for (int i=1;i<n;i++){
        if (s[i]!=s[i-1]){
            d2[i]=-1;
            continue;
        }
        for (k=1;i-k-1>=0 && i+k<n;k++) if (s[i-k-1]!=s[i+k]) break;
        d2[i]=max(k-1,0);
    }
    int *k1=max_element(d1,d1+n),*k2=max_element(d2+1,d2+n);
    if (*k1>=*k2+1) for (int i=k1-d1-*k1;i<=k1-d1+*k1;i++) cout<<s[i];
    else for (int i=k2-d2-*k2-1;i<=k2-d2+*k2;i++) cout<<s[i];
     return 0;
}

Can I get this test?

a

you must output
a