File: reformat-date/solution.py

Date: 2026-06-06

Time: 18:41

reformat-date/solution.py

Purpose

This file solves LeetCode #1507: Reformat Date. It converts a human-readable date string in "Day Month Year" format (e.g., "20th Oct 2052") into the ISO-like "YYYY-MM-DD" format (e.g., "2052-10-20"). The file is self-contained: it holds both the solution class and its unit tests.

Key Components

Solution.reformatDate(date: str) -> str — The core method. It:

1. Defines a months dict mapping 3-letter abbreviations to zero-padded month numbers.

2. Splits the input into three tokens: day (with ordinal suffix), month abbreviation, year.

3. Strips the ordinal suffix from the day using rstrip("stndrdth").

4. Returns the formatted string with zfill(2) to ensure single-digit days get a leading zero.

TestReformatDate — Three test methods covering:

• test_examples: The three examples from the LeetCode problem statement.
• testordinalsuffixes: All four ordinal suffix types (st, nd, rd, th) including edge cases like 21st, 22nd, 23rd, 31st.
• testallmonths: Iterates all 12 months to verify correct month-number mapping.

Patterns

• Single-file solution+test: Standard pattern in this repo — every problem directory bundles solution.py with an inline unittest suite and a separate test_solution.py that imports from it.
• Dictionary lookup over parsing: Uses a static dict for month mapping rather than datetime.strptime, keeping the solution self-contained and O(1).
• rstrip for suffix removal: A concise idiom — rstrip("stndrdth") strips any trailing characters from the set {s, t, n, d, r, h}. This works because no valid day digit (0-9) appears in that character set, so it cleanly removes "st", "nd", "rd", or "th" without touching the numeric portion.

Dependencies

Imports: Only unittest from the standard library — no external dependencies.

Imported by: The test_solution.py in the same directory imports Solution from this file. The large "Imported By" list in the prompt is misleading — those are *other* problems' test files, not actual importers of this module. They share the same structural pattern but don't reference this file's code.

Flow

"20th Oct 2052"
  → split() → ["20th", "Oct", "2052"]
  → rstrip("stndrdth") on "20th" → "20"
  → months["Oct"] → "10"
  → f"{2052}-{10}-{20}" → "2052-10-20"

For single-digit days like "6th": rstrip yields "6", then zfill(2) pads to "06".

Invariants

• Input is guaranteed to be well-formed per the LeetCode constraint: always "Day Month Year" with a valid ordinal suffix and 3-letter month abbreviation.
• The months dict covers exactly the 12 valid month abbreviations. An unrecognized abbreviation would raise KeyError.
• rstrip("stndrdth") is safe only because day digits 0-9 are disjoint from the strip character set {s,t,n,d,r,h}.

Error Handling

None. The solution trusts LeetCode's input guarantees. An invalid month abbreviation would propagate a KeyError; a completely non-numeric day string would produce garbage from zfill but not raise.

Beliefs

• rstrip-suffix-safety — rstrip("stndrdth") never removes day digits because no digit character appears in the strip set {s,t,n,d,r,h}
• month-dict-completeness — The months dictionary maps exactly the 12 three-letter English month abbreviations to zero-padded two-digit strings "01" through "12"
• zfill-single-digit-padding — zfill(2) is only needed for days 1–9; days 10–31 already have two characters after suffix stripping
• no-stdlib-date-parsing — The solution avoids datetime entirely, relying on manual string splitting and dictionary lookup