小さい方のcombination

combination.cpp

  long long combination(int n, int r){
      long long i = 1;
      int k = r>n-r?n-r:r, cnt;
      if(k < 0) return 0;
      for(cnt = 1; cnt <= k; cnt++){
        i *= n - cnt + 1;
        i /= cnt
      }
      return i;
  }