hello.cc

// https://developers.google.com/edu/c++/getting-started

#include <iostream>
#include <cassert>
using namespace std;

static int a, b, c;

int new_sum(int sum, bool& terminated)
{
  if (sum % 2 == 1) {
    if (sum == 9) {
      terminated = true;
      // seems unreachable at all...
      assert(0);
      return 0;
    }
    if (sum + 11 > 0 && sum + 11 < 20) {
      return sum+11;
    } else if (sum-11 > 0 && sum - 11 < 20) {
      return sum-11;
    } else {
      cout << "error" << sum << endl;
      assert(!"unreachable");
    }
  }
  return sum;
}

int main() {
  for (a = 1; a <= 9; ++a) {
    for (b = 1; b <= 9; ++b) {
      for (c = 1; c <= 9; ++c) {
        int o_a = a;
        int o_b = b;
        int o_c = c;

        for (int i = 0; i < 1; ++i) {
          int abc = a*100 + b*10 + c*1;
          int bca = b*100 + c*10 + a*1;
          int cab = c*100 + a*10 + b*1;
          int x = abc % 11;
          int y = bca % 11;
          int z = cab % 11;

          bool terminated = false;
          int sum;
          sum = new_sum(x+y, terminated);
          if (terminated) {
            break;
          }
          a = sum/2;

          sum = new_sum(y+z, terminated);
          if (terminated) {
            break;
          }
          b = sum/2;

          sum = new_sum(z+x, terminated);
          if (terminated) {
            break;
          }
          c = sum/2;
        }

        if (a != o_a || b != o_b || c != o_c) {
          cout << o_a << o_b << o_c << endl;
          cout << a << b << c << endl;
          return 0;
        }

      }
    }
  }
}