Count Days Spent Together — LeetCode 2409 Python Solution

The problem

Alice and Bob are traveling to Rome for separate business meetings. You are given 4 strings arriveAlice, leaveAlice, arriveBob, and leaveBob.

Example

Input

arriveAlice = "08-15", leaveAlice = "08-18", arriveBob = "08-16", leaveBob = "08-19"

Output

3

Explanation

Alice will be in Rome from August 15 to August 18. Bob will be in Rome from August 16 to August 19. They are both in Rome together on August 16th, 17th, and 18th, so the answer is 3.

Python solution

class Solution:
    def countDaysTogether(
        self, arriveAlice: str, leaveAlice: str, arriveBob: str, leaveBob: str
    ) -> int:
        a = max(arriveAlice, arriveBob)
        b = min(leaveAlice, leaveBob)
        days = (31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31)
        x = sum(days[: int(a[:2]) - 1]) + int(a[3:])
        y = sum(days[: int(b[:2]) - 1]) + int(b[3:])
        return max(y - x + 1, 0)

Complexity

Pattern: Math and Number Theory

Find the closed form, the invariant, or the modular identity — and skip the loop entirely. LeetCode 2409. Count Days Spent Together is filed here on both counts: the reference solution below belongs to the algorithm family this hub collects, and LeetCode tags it Math.

The math and number theory guide has the Python template for the pattern and the 485 LeetCode problems that use it.

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