Language: C++

#include <bits/stdc++.h>
#define rep(i, n) for(int i = 0; i < n; i++)
using namespace std;

int main() {
    long n, k, ans;
    string str;
    cin >> str >> k;
    n = str.length();
    if(k == 1) {
        ans = (str.at(0) - '0') + (n - 1) * 9;
    } else if(k == 2) {
        ans = (n - 1) * (n - 2) * 81 / 2;
        bool flag = true;
        rep(i, n - 1) {
            if(flag) ans += (str.at(0) - '1') * 9 + (str.at(i + 1) - '0');
            else ans += (str.at(0) - '0') * 9;
            if(str.at(i + 1) != '0') flag = false;
        }
    } else {
        ans = (n - 1) * (n - 2) * (n - 3) * 243 / 2;
        ans = max(0, ans);
        bool flag = true, fflag = true;
        rep(i, n - 1) {
            rep(j, n - 1) {
                if(i >= j) continue;
                if(flag) {
                    if(fflag) ans += (str.at(0) - '1') * 81 + (str.at(i + 1) - '1') * 9 + (str.at(j + 1) - '0');
                    else ans += (str.at(0) - '1') * 81 +(str.at(i + 1) - '0') * 9;
                    if(str.at(j + 1) != '0') fflag = false;
                } else {
                    ans += (str.at(0) - '0') * 81;
                }
            }
            if(str.at(i) != '0') flag = false;
        }
    }
    cout << ans << endl;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:22:18: error: no matching function for call to 'max(int, long int&)'
   22 |         ans = max(0, ans);
      |               ~~~^~~~~~~~
In file included from /usr/include/c++/14/algorithm:60,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:51,
                 from a.cc:1:
/usr/include/c++/14/bits/stl_algobase.h:257:5: note: candidate: 'template<class _Tp> constexpr const _Tp& std::max(const _Tp&, const _Tp&)'
  257 |     max(const _Tp& __a, const _Tp& __b)
      |     ^~~
/usr/include/c++/14/bits/stl_algobase.h:257:5: note:   template argument deduction/substitution failed:
a.cc:22:18: note:   deduced conflicting types for parameter 'const _Tp' ('int' and 'long int')
   22 |         ans = max(0, ans);
      |               ~~~^~~~~~~~
/usr/include/c++/14/bits/stl_algobase.h:303:5: note: candidate: 'template<class _Tp, class _Compare> constexpr const _Tp& std::max(const _Tp&, const _Tp&, _Compare)'
  303 |     max(const _Tp& __a, const _Tp& __b, _Compare __comp)
      |     ^~~
/usr/include/c++/14/bits/stl_algobase.h:303:5: note:   candidate expects 3 arguments, 2 provided
In file included from /usr/include/c++/14/algorithm:61:
/usr/include/c++/14/bits/stl_algo.h:5706:5: note: candidate: 'template<class _Tp> constexpr _Tp std::max(initializer_list<_Tp>)'
 5706 |     max(initializer_list<_Tp> __l)
      |     ^~~
/usr/include/c++/14/bits/stl_algo.h:5706:5: note:   candidate expects 1 argument, 2 provided
/usr/include/c++/14/bits/stl_algo.h:5716:5: note: candidate: 'template<class _Tp, class _Compare> constexpr _Tp std::max(initializer_list<_Tp>, _Compare)'
 5716 |     max(initializer_list<_Tp> __l, _Compare __comp)
      |     ^~~
/usr/include/c++/14/bits/stl_algo.h:5716:5: note:   template argument deduction/substitution failed:
a.cc:22:18: note:   mismatched types 'std::initializer_list<_Tp>' and 'int'
   22 |         ans = max(0, ans);
      |               ~~~^~~~~~~~