problem_19.py

"""
Problem 19: Counting Sundays

You are given the following information, but you may prefer
to do some research for yourself.

- 1 Jan 1900 was a Monday.
- Thirty days has September, April, June and November.
All the rest have thirty-one, Saving February alone, Which
has twenty-eight, rain or shine. And on leap years, twenty-nine.
- A leap year occurs on any year evenly divisible by 4, but
not on a century unless it is divisible by 400.

How many Sundays fell on the first of the month during the
twentieth century (1 Jan 1901 to 31 Dec 2000)?
"""

sundays = 0
day = 1  # monday
for year in range(1900, 2001):
    months = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]
    
    if year % 4 == 0 and year != 1900:
        months[1] = 29
    
    for month in months:
        day = month + day
        if day % 7 == 0 and year != 1900:
            sundays += 1

print(sundays)