数组不会越界，用的STL，求助SOS

测试信息

#include<bits/stdc++.h>
#define ll long long
#define pb push_back
#define ve vector
#define endl '\n'
#define itn int
using namespace std;
deque<int> arr[100050];
int main()
{
    ios::sync_with_stdio(0),cin.tie(0),cout.tie(0);
    //begin
    int q;
    cin>>q;
    for(int i=0;i<q;i++){
    	string s;
    	cin>>s;
    	if(s=="push_back"){
    		int a,x;
    		cin>>a>>x;
    		arr[a].push_back(x);
		}
		if(s=="pop_back"){
			int a;
			cin>>a;
			if(!arr[a].empty())
				arr[a].pop_back();
		}
		if(s=="push_front"){
			int a,x;
			cin>>a>>x;
			arr[a].push_front(x);
		}
		if(s=="pop_front"){
			int a;
			cin>>a;
			if(!arr[a].empty())
				arr[a].pop_front();
		}
		if(s=="size"){
			int a;
			cin>>a;
			cout<<arr[a].size()<<endl;
		}
		if(s=="front"){
			int a;
			cin>>a;
			if(!arr[a].empty())
				cout<<arr[a].front()<<endl;
		}
		if(s=="back"){
			int a;
			cin>>a;
			if(!arr[a].empty())
				cout<<arr[a].back()<<endl;
		}
	}
	//end
    return 0;
}