全WA求助，是不是哪里漏判了

#include<bits/stdc++.h>
using namespace std;
inline int read()
{
	char c=getchar();int x=0,f=1;
	for(;!isdigit(c);c=getchar())if(c=='-')f=-1;
	for(;isdigit(c);c=getchar())x=x*10+c-48;
	return x*f;
}
int n; 
list<long long>q[2000010];
int main()
{
	ios::sync_with_stdio(false);
	n=read();
	while(n--)
	{
		string s;
		cin>>s;
		if(s=="push_back")
		{
			long long a,b;
			cin>>a>>b;
			q[a].push_back(b);
		}
		else if(s=="push_front")
		{
			long long a,b;
			cin>>a>>b;
			q[a].push_front(b);		
		}
		else if(s=="pop_back")
		{
			long long a;
			cin>>a;
			if(!q[a].empty())
			{
				q[a].pop_back();
			}
		}
		else if(s=="pop_front")
		{
			long long a;
			cin>>a;
			if(!q[a].empty())
			{
				q[a].pop_front();
			}
		}
		else if(s=="size")
		{
			long long a;
			cin>>a;
			cout<<q[a].size()<<'\n';
		}
		else if(s=="front")
		{
			long long a;
			cin>>a;
			if(!q[a].empty())
			{
				cout<<q[a].front()<<'\n';
			}
		}
		else if(s=="back")
		{
			long long a;
			cin>>a;
			if(!q[a].empty())
			{
				cout<<q[a].back()<<'\n';
			}			
		}		
	}
	return 0;
}