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)?

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Idea

Accumulate the total days of each month since 1 Jan 1900, and check if the first day of this month is sunday.

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month_day_norm = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]
month_day_leap = [31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]

def is_leap(y):
    return y % 400 == 0 or (y % 100 != 0 and y % 4 == 0)

is_leap(1901), is_leap(1904), is_leap(2000), is_leap(1900)

(False, True, True, False)

def solve():
    day_cnt = 0
    sunday_first_cnt = 0
    for y in range(1900, 2001):
        if is_leap(y):
            month_day = month_day_leap
        else:
            month_day = month_day_norm
            
        for m in range(12):
            if y != 1900 and day_cnt % 7 == 6:
                sunday_first_cnt += 1
            day_cnt += month_day[m]
    return sunday_first_cnt

solve()

171