Comet OJ - Contest #12 补题

#include <bits/stdc++.h>
typedef long long ll;
const int maxn = 1e5+7;
using namespace std;
string s,s1;
set<string>ss; 
int main(){
    int T;
    cin>>T;
    while(T--){
        int n;
        cin>>n;
        ss.clear();
        int flag = 0;
        for(int i=0;i<n;i++){
            cin>>s;
            s1 = s.substr(0,3);
            if(flag==0&&ss.count(s1)){
                flag = 1;
            }
            else ss.insert(s1);
        }
        if(flag) cout<<"No"<<endl;
        else cout<<"Yes"<<endl;
    }
    return 0;
}