题目

样例对了但全WA。

#include<bits/stdc++.h>
using namespace std;
int main(){
	queue<int> q;
	int n;
	cin>>n;
	while(n--){
		int op;
		cin>>op;
		switch(op){
		case 1:
			int x;
			cin>>x;
			q.push(x);
			break;
		case 2:
			if(q.empty()==true) printf("ERR_CANNOT_QUERY\n");
			else q.pop();
			break;
		case 3:
			if(q.empty()==true) printf("ERR_CANNOT_QUERY\n");
			else printf("%d\n",q.front());
			break;
		case 4:
			printf("%d\n",q.size());
			break;
		}	
	}
	return 0;
}