019.cpp

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#include <iostream>
using namespace std;

unsigned short
weekday (int DD, int MM, int YYYY)
{
//Jan & Feb are counted as month 13 & 14 of the prevoius year
  if (MM <= 2)
    {
      MM += 12;
      YYYY--;
    }
  return
    (YYYY + YYYY / 4 + (YYYY / 100) * 6 + YYYY / 400 + 26 * (MM + 1) / 10 +
     DD - 1) % 7;
}

int
main ()
{
  unsigned count = 0;
  for (int yyyy = 1901; yyyy <= 2000; yyyy++)
    for (int mm = 1; mm <= 12; mm++)
      if (weekday (1, mm, yyyy) == 0)   //Sunday
        {
          count++;
          //cout << mm <<'/'<<yyyy<<endl;
        }
  cout << count << endl;
  return 0;
}