感觉没问题，交上去直接 MLE，求大佬指点：

#include<bits/stdc++.h>
using namespace std;
const int N=1e6+1;
int T,a,x;
string s;
deque<int>q[N];
int main(){
    cin>>T;
    while(T--){
    	cin>>s;
    	if(s=="push_back"){
    		cin>>a>>x;
    		q[a].push_back(x);
		}
		if(s=="pop_back"){
			cin>>a;
			if(q[a].size()) q[a].pop_back();
		}
		if(s=="push_front"){
    		cin>>a>>x;
    		q[a].push_front(x);
		}
		if(s=="pop_front"){
			cin>>a;
			if(q[a].size()) q[a].pop_front();
		}
		if(s=="size"){
			cin>>a;
			cout<<q[a].size()<<endl;
		}
		if(s=="front"){
			cin>>a;
			if(q[a].size()) cout<<q[a].front()<<endl;
		}
		if(s=="back"){
			cin>>a;
			if(q[a].size()) cout<<q[a].back()<<endl;
		}
	}
}