根本找不出问题！

#include<iostream>
#include<list>
using namespace std;
list<int> deque[1000005];
string s;
int q, a, x;
int main(){
    cin >> q;
    while (q--){
        cin >> s >> a;
        if (s == "push_back") {
            cin >> x;
            deque[a].push_back(x);
        }
        else if (s == "pop_back")
            if (deque[a].size()) deque[a].pop_back();
        else if (s == "push_front"){
            cin >> x;
            deque[a].push_front(x);
        }
        else if(s=="pop_front")
            if (deque[a].size()) deque[a].pop_front();
        else if(s=="size") 
            cout << deque[a].size() << endl;
        else if(s=="front")
            if (deque[a].size()) cout << deque[a].front() << endl;
        else if(s == "back")
            if (deque[a].size()) cout << deque[a].back() << endl;
    }
    return 0;
}