1360F - Spy-string - CodeForces Solution

#include <bits/stdc++.h>
#include <ext/numeric>
#include <ext/pb_ds/assoc_container.hpp>
#define fast cin.tie(0); cin.sync_with_stdio(0);
#define ll long long
#define ull unsigned long long
#define ld long double
#define oo (ll)1e9
#define OO (ll)1e18
#define PI acos(-1)
#define LL __int128
using namespace std;
using namespace __gnu_pbds;
template<class T> using ordered_set = tree<T, null_type , less<T> , rb_tree_tag , tree_order_statistics_node_update> ;
template<class T> using ordered_multiset = tree<T, null_type, less_equal<T>,rb_tree_tag, tree_order_statistics_node_update>;
const ll mod = 1e9 + 7;
const int MOD = 998244353;
const ld eps = 1e-6;
int tt = 1;
int test;
void solve()
{
     int n , m;
     cin >> n >> m;
     string a[n];
     for(int i = 0 ; i < n ; i++) cin >> a[i];
     auto isGood = [&](string x ,string y)
     {
         int cnt = 0;
         for(int i = 0 ; i < m ; i++) cnt += (x[i] != y[i]);
         return cnt <= 1;
     };
     for(int i = 0 ; i < m ; i++) {
        string myOne = a[0];
        for(char j = 'a' ; j <= 'z' ; j++) {
            myOne[i] = j;
            bool ok = 1;
            for(int k = 1 ; k < n ; k++) {
                ok &= isGood(myOne , a[k]);
            }
            if(ok) {
                cout << myOne << '\n';
                return;
            }
        }
     }
     cout << -1 << '\n';
}
int main()
{
    fast
    cin >> tt;
    while(tt--)
        solve();
    return 0;
}