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Problem Statement

Given two dates date1 and date2 in the format YYYY-MM-DD, return the number of days between the two dates.

Clarifying Questions

  1. Are the dates always valid (e.g., checking for leap years, correct month and day ranges)?
  2. Shall we consider the date order? For example, if date1 is after date2, should we return a positive number of days?
  3. Is it guaranteed that the input format is always YYYY-MM-DD?

We’ll assume the input format is always valid and as specified.

Strategy

We can leverage Python’s datetime module to handle date manipulations and calculations easily. Here’s the plan:

  1. Parse the input dates using datetime.strptime.
  2. Calculate the difference between the two dates.
  3. Return the absolute value of the difference in days.

Code

Here’s a Python function to achieve this:

from datetime import datetime

def days_between_dates(date1: str, date2: str) -> int:
    # Define the date format
    date_format = "%Y-%m-%d"
    
    # Parse the input dates
    d1 = datetime.strptime(date1, date_format)
    d2 = datetime.strptime(date2, date_format)
    
    # Calculate the difference in days
    delta = abs((d2 - d1).days)
    
    return delta

# Example usage:
date1 = "2020-01-15"
date2 = "2019-12-31"
print(days_between_dates(date1, date2))  # Output: 15

Time Complexity

The time complexity of this solution is (O(1)) since parsing the dates and computing the difference is done in constant time with respect to the input size.

Explanation

  1. The datetime.strptime method parses the input string into a datetime object, making it easier to handle date arithmetic.
  2. The difference between two datetime objects returns a timedelta object, from which we can extract the number of days.
  3. Using abs ensures that we always return a positive number of days regardless of the order of the input dates.

This solution should cover typical use cases efficiently and correctly. If additional constraints or edge cases need to be considered, further clarifications can be added.

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