完善程序(1)处应填()。

#include<bits/stdc++.h>

#define ll long long long

using namespace std;

ll n, num, mid, t, a[1000001], b[1000001];

int main(){

    cin >> n;

    for (int i = 1; i <= n; i++) cin >> a[i];

    int i = n;

    while (i > 1) {

        if (__①____)

            num++;

            b[num] = a[i];

        }

        else{

            num++;

            b[num] = a[i];

            num++;

            ___②__;

            mid = i - 2;

            break;

        }

        i--;

    }

    if (i == 1) __③__;

    else{

        for (i = 1; i <= num - 1; i++)

            if (___④___)

                t = b[i];

                b[i] = b[num];

                b[num] = t;

                break;

    }

    for (i = 1; i <= mid; i++) cout << a[i] << " ";

    cout << b[num] << " ";

    for (i = num + 1; i++) __⑤__;

    cout << endl;

}