南园十三首·其五

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输入样例1：

5
1 2
1 3
3
2
2

输出样例1：

0
1
1
0
-1

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输入样例2：

4
1 2
3
1 3
3

输出样例2：

0
0
1
1

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C/C++ 

#include<bits/stdc++.h>
using namespace std;
typedef long long LL;
list<LL> line;
bool flag = true;
int main()
{
    LL q,a,b;
    scanf("%lld",&q);
    while (q--){
        scanf("%lld",&a);
        if(a==1){
            scanf("%lld",&b);
            if(flag) line.push_back(b);
            else line.push_front(b);
        }
        else if(a==2){
            if(!line.empty()) {
                if(flag) line.pop_front();
                else line.pop_back();
            }
        }
        else flag = !flag;
 
        if(line.empty()) cout << "-1" << endl;
        else
        {
            if(flag)
                cout << (line.front()^line.back()) << endl;
            else cout << (line.back()^line.front()) << endl;
        }
    }
 
    return 0;
}

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