【題解】AtCoder ABC176C – Step

#include <iostream>
using namespace std;

int n, a[200005];
long long ans;

int main() {
    cin >> n;
    for (int i = 1; i <= n; i++){
        cin >> a[i];
        if (a[i] < a[i-1]){
            ans += a[i-1]-a[i];
            a[i] = a[i-1];
        }
    }
    cout << ans << "\n";
}