【題解】Codeforces 1392C. Omkar and Waterslide

#include <iostream>
using namespace std;
 
int t, n, a[200005];
long long ans;
 
int main() {
    cin >> t;
    while (t--){
        cin >> n;
        cin >> a[0];
        ans = 0;
        for (int i = 1; i < n; i++){
            cin >> a[i];
            if (a[i-1] > a[i]) ans += a[i-1]-a[i];
        }
        cout << ans << "\n";
    }
}