Binary Watch Python Solution with Tests

Tested Python solution for LeetCode 401 with 16 pytest cases. Generate a practice environment with lcpy.

LeetCode 401, Easy. Topics: Backtracking, Bit Manipulation. View on LeetCode.

Generate this problem as a practice environment: tested reference solution, 16 parametrized pytest cases, and a playground notebook:

lcpy gen -n 401   # by problem number
lcpy gen -s binary_watch   # by problem name

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Problem

A binary watch has 4 LEDs on the top to represent the hours (0-11), and 6 LEDs on the bottom to represent the minutes (0-59). Each LED represents a zero or one, with the least significant bit on the right.

For example, the below binary watch reads "4:51".

Given an integer turnedOn which represents the number of LEDs that are currently on (ignoring the PM), return all possible times the watch could represent. You may return the answer in any order.

The hour must not contain a leading zero.

• For example, "01:00" is not valid. It should be "1:00".

The minute must consist of two digits and may contain a leading zero.

• For example, "10:2" is not valid. It should be "10:02".

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Examples

Input: turnedOn = 1
Output: ["0:01","0:02","0:04","0:08","0:16","0:32","1:00","2:00","4:00","8:00"]

Input: turnedOn = 9
Output: []

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Constraints

• 0 <= turnedOn <= 10

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Solution

Reference implementation from solution.py on GitHub, full suite in test_solution.py:

class Solution:
    # Time: O(12 * 60)
    # Space: O(1) excluding the output
    def read_binary_watch(self, turned_on: int) -> list[str]:
        if turned_on > 9:
            return []
        result: list[str] = []
        for hour in range(12):
            for minute in range(60):
                if hour.bit_count() + minute.bit_count() == turned_on:
                    result.append(f"{hour}:{minute:02d}")
        return result

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Complexity

Time	Space
O(12 * 60)	O(1) excluding the output

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