题解 | 【模板】序列操作，这题可以经常看看记忆。注意从循环用的字母不要和变量一样，容易混淆

#include <algorithm>
#include <functional>
#include <iostream>
#include <vector>
using namespace std;

int main() {
    int q;
    cin>>q;
    int i;
    int x;
    int n;
    vector<int> arr;

    for (int k=0; k<q; k++) {
        cin>>n;
        switch (n) {
        case 1:
            cin>>x;
            arr.push_back(x);
            break;
        case 2:
        if (!arr.empty()) {
            arr.pop_back();
        }
            break;
        case 3:
            cin>>i;
            cout<<arr[i]<<endl;
            break;
        case 4:
            cin>>i>>x;
            arr.insert(arr.begin()+i+1, x);
            break;
        case 5:
            sort(arr.begin(), arr.end());
            break;
        case 6:
            sort(arr.begin(), arr.end(), greater());
            break;
        case 7:
            cout<<arr.size()<<endl;
            break;
        case 8:
            for (int j : arr) {
                cout<<j<<" ";
            }
            cout<<endl;
            break;
        }

        // cout<<k<<endl;
    }
}