244B - Undoubtedly Lucky Numbers - CodeForces Solution

#include <bits/stdc++.h>
#include <map>
using namespace std;
long long int ans;
int n, a, b;
std::map<int, int> S;
void DFS(long long int value, int length)
{
  if (value > n || length > 10)
    return;
  S[value]++;
  DFS(value * 10 + a, length + 1);
  DFS(value * 10 + b, length + 1);
}
int main()
{

  cin >> n;
  ans = 0;
  for (a = 0; a < 10; a++)
    for (b = a; b < 10; b++)
      DFS(0, 0);
  cout << S.size() - 1; // delete 0

  return 0;
}