Date: 2026-06-06
Time: 17:20
This file solves LeetCode 1736 — Latest Time by Replacing Hidden Digits. Given a time string in "HH:MM" format where ? characters represent unknown digits, it finds the latest (maximum) valid time by greedily replacing each ? with the largest digit that keeps the time valid. The file is self-contained: solution class and unit tests in one module.
Solution.maximumTime(self, time: str) -> strThe single method. It converts the input string to a mutable list, then resolves each ? position left-to-right with greedy choices:
| Position | Rule | Rationale |
|----------|------|-----------|
| t[0] (tens-hour) | '2' if t[1] is ? or 0-3; else '1' | Hours go up to 23. If the ones-hour is 4–9, tens-hour can't be 2 (that would exceed 23), so 1 is the max. |
| t[1] (ones-hour) | '3' if t[0] == '2'; else '9' | When tens-hour is 2, ones-hour maxes at 3 (23:xx). Otherwise it's 9 (19:xx, 09:xx). |
| t[3] (tens-minute) | '5' | Minutes range 00–59; tens digit maxes at 5. |
| t[4] (ones-minute) | '9' | Ones digit always maxes at 9. |
Position t[2] is always : and is never touched.
TestMaximumTimeTwelve test cases covering: the three LeetCode examples, all-?, no-?, each individual position in isolation, and the critical coupling between t[0] and t[1] (the ?4:00 → 14:00 vs ?3:00 → 23:00 distinction).
Greedy positional resolution. Each digit is resolved independently, left-to-right. This works because earlier positions have strictly higher place value — maximizing them first always produces the global maximum. The only dependency is between positions 0 and 1 (hours), which the code handles via cross-referencing.
Mutable list idiom. Python strings are immutable, so the code converts to list(time), mutates in place, then joins back. Standard pattern for character-level string manipulation in Python.
Self-contained test module. Tests live alongside the solution with unittest.main() at the bottom, matching the repo-wide convention.
Imports: Only unittest from the standard library. No external dependencies.
Imported by: The test_solution.py file in this same directory imports from it. The massive "Imported By" list in the prompt is misleading — those are *other problems'* test files that happen to share the same structural pattern, not actual importers of this module.
1. Input: "HH:MM" string with ? wildcards
2. Convert to list of 5 characters
3. Resolve t[0] — depends on t[1]'s current value (which may still be ?)
4. Resolve t[1] — depends on t[0]'s now-resolved value
5. Resolve t[3] — independent
6. Resolve t[4] — independent
7. Join and return
The ordering matters: t[0] must be resolved before t[1] because t[1]'s max depends on whether t[0] is 2. Conversely, t[0]'s logic checks t[1]'s *original* value (or ?), which is correct — when t[1] is still ?, it means "any digit is possible," so t[0] can safely be 2.
00:00 through 23:59).? is replaced with the largest digit that maintains validity.? characters are never modified.t[0] and t[1]; the minute constraint (0–59) does not create any cross-position dependency.None. The method assumes well-formed input: exactly 5 characters, format "??:??" where each ? is either a valid digit or ?, and a colon at position 2. No validation, no exceptions. This is typical for LeetCode solutions where input constraints are guaranteed by the problem.
latest-time-by-replacing-hidden-digits/test_solution.py — Separate test file that imports this solution; check whether it duplicates or extends the inline testsnumber-of-valid-clock-times/solution.py — Related problem (counting valid times instead of maximizing); likely uses similar digit-constraint logicgreedy-digit-resolution — The pattern of resolving digits left-to-right by place value appears in several problems (maximum-69-number, largest-number-after-digit-swaps-by-parity); compare approacheslatest-time-by-replacing-hidden-digits/review.md — Code review notes that may document edge cases or alternative approaches consideredlatest-time-by-replacing-hidden-digits/solution.py:maximumTime — The t[1] in '?0123' check is the subtle heart of the algorithm; trace what happens when t[1] is 4 vs 3 to build intuitiongreedy-left-to-right-is-optimal — Resolving digits left-to-right (highest place value first) produces the globally maximum valid time because each position's contribution strictly dominates all lower positions combined.hour-digits-are-coupled — Position 0 and position 1 have a mutual dependency: t[0]'s max depends on t[1]'s current value, and t[1]'s max depends on t[0]'s resolved value. The code handles this by resolving t[0] first while t[1] may still be ?.minute-digits-are-independent — Positions 3 and 4 have fixed maximums (5 and 9) regardless of any other position, because the minute range 00–59 doesn't create cross-digit constraints.question-mark-means-unconstrained-for-predecessor — When t[0] checks t[1] in '?0123', including ? is correct: if t[1] is unknown, t[0]=2 is safe because t[1] will later resolve to at most 3.