手动实现循环队列

#include <bits/stdc++.h>
using namespace std;
int n;
int op, x;

const int MAX_SIZE = 100;
int q[MAX_SIZE], head, tail, sz;

int main()
{
    cin >> n;

    head = 0, tail = -1, sz = 0;

    for (int i = 1; i <= n; i++)
    {
        cin >> op >> x;
        if (op == 1)
        {
            tail = (tail + 1) % MAX_SIZE;
            q[tail] = x;
            sz++;
        }
        else if (op == 2)
        {
            if (sz > 0)
                cout << q[head] << "\n";
            else
                cout << -1 << "\n";
        }
        else if (op == 3)
        {
            if (sz > 0)
            {
                head++;
                sz--;
            }
        }
    }
    return 0;
}

STL 实现

#include <bits/stdc++.h>
using namespace std;
int n;
int op, x;
queue<int> q;

int main()
{
    cin >> n;
    for (int i = 1; i <= n; i++)
    {
        cin >> op >> x;
        if (op == 1)
        {
            q.push(x);
        }
        else if (op == 2)
        {
            if (q.size() > 0)
                cout << q.front() << "\n";
            else
                cout << -1 << "\n";
        }
        else if (op == 3)
        {
            if (!q.empty())
                q.pop();
        }
    }
    return 0;
}