将数字0~19存入一个整型数组a中(即a[0]=0……a[19]=19)，然后再逆序输出

Sure, here's the code to store the numbers 0 ~ 19 in an integer array "a" and then print them out in reverse order:

#include <iostream>

using namespace std;

int main() {
    int a[20];  // declare an integer array with size 20
    for (int i = 0; i < 20; i++) {
        a[i] = i;  // store the numbers 0 ~ 19 in the array
    }

    // print out the array in reverse order
    for (int i = 19; i >= 0; i--) {
        cout << a[i] << " ";
    }
    cout << endl;

    return 0;
}

Hope this helps!