After the first timeout, it's a quick idea to look at the question again. Just mark it with array subscripts.
But there was a slight mistake.
1. Note that it's a prime, not an odd number.
2. After the change, there is still a point that can not be divided, after several investigations.
When I judge whether this number is in the array, judgment is not equal to - 1, judgment is not passable, judgment is 1 passed, do not know what the exact number of this sample test is? Normally, this number is not 1. That must be - 1. Ah, confused.

#include <cstdio>
#include <cstdlib>
#include <algorithm>
#include <iostream>
#include <string>
#include <cctype>
#include <vector>
#include <cstring>
#include <cmath>
using namespace std;

struct student{
	string id;
	string award;
	int rank;
	int check;
};
int n,m;

int isprime(int a){
	int b = sqrt((double)a);
	for(int i = 2;i <= b;i++){
		if(a%i==0){
			return 0;
		}
	}
	return 1;
}

int main()
{
 	cin >> n;
 	vector<student> stu(10000);
 	int ans[10000];
 	memset(ans,-1,10000);
	for(int i=1; i<=n; i++){
		string a;
		cin >> a;
		int num = stoi(a);
		ans[num] = 1;
		stu[num].id = a;
		stu[num].rank = i;
		if(i==1) stu[num].award = "Mystery Award";
		else if(isprime(i)) stu[num].award = "Minion";
		else stu[num].award = "Chocolate";
	}	
 	cin >> m; 
 	for(int i=0; i<m; i++){
 		string a;
 		cin >> a;
 		int num = stoi(a);
 		if(ans[num]==1){
 			if(stu[num].check==0){
 				cout << a <<": "<< stu[num].award<< endl;
 			   stu[num].check = 1;
			}
			else cout << a <<": Checked"<<endl;
		}
		else cout << a << ": Are you kidding?"<< endl;
 		
	 }
  	
    return 0;
}

Timeout code

#include <cstdio>
#include <cstdlib>
#include <algorithm>
#include <iostream>
#include <string>
#include <cctype>
#include <vector>
#include <cstring>
#include <map>
using namespace std;

struct student{
	string id;
	string award;
	int rank;
	int check;
};
int n,m;


int main()
{
 	cin >> n;
 	vector<student> stu(n+1);
	for(int i=1; i<=n; i++){
		string a;
		cin >> a;
		stu[i].id = a;
		stu[i].rank = i;
		if(i==1) stu[i].award = "Mystery Award";
		else if(i%2!=0) stu[i].award = "Minion";
		else stu[i].award = "Chocolate";
	}	
 	cin >> m; 
 	for(int i=0; i<m; i++){
 		string a;
 		cin >> a;
 		int flag = 0;
 		for(int j=1; j<=n; j++){			
 			//cout << stu[j].id << " a " << stu[j].check << endl;
 			if(a==stu[j].id&&stu[j].check==0){
 				cout << a <<": "<< stu[j].award<< endl;
 				stu[j].check = 1;
 				flag = 1;
 				break;
			}
			if(a==stu[j].id&&stu[j].check==1){
				cout << a <<": Checked"<<endl;
				flag = 1;
				break;
			} 
		 }
		 if(flag==0) cout << a << ": Are you kidding?"<< endl;
	 }
 	
 	
    return 0;
}