Deleting digits

#include <bits/stdc++.h>
using namespace std;

typedef unsigned long long ull;

int main(){
    string n;
    std::cin >> n;
    ull nn = stoull(n);

    int r=0;
    for(; n != "0"; r++){
        char d = *max_element(n.begin(), n.end());
        nn -= d - '0';
        n = to_string(nn);
    }

    cout << r << endl;
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: ACCEPTED

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED

Test 11

Verdict: ACCEPTED

Test 12

Verdict: ACCEPTED

Test 13

Verdict: ACCEPTED