missing-number.cpp

#include <iostream>

using namespace std;

int main() {
  const int N = 10;
  int numbers[] = {0, 2, 1, 3, 10, 4, 6, 7, 8, 5};
  int answer = 0;

  for (int bit = 0; (1 << bit) <= N; ++bit) {
    int cnt_expected = 0, cnt_actual = 0;
    for (int i = 0; i <= N; ++i) {
      cnt_expected += (i & (1 << bit)) != 0;
    }
    for (auto i: numbers) {
      cnt_actual += (i & (1 << bit)) != 0;
    }
    if (cnt_expected > cnt_actual) {
      answer |= 1 << bit;
    }
  }
  cout << answer << endl;
}