Binary Watch

Question

A binary watch has 4 LEDs on the top which represent the hours (0-11), and the 6 LEDs on the bottom represent the minutes (0-59).

Each LED represents a zero or one, with the least significant bit on the right.

For example, the above binary watch reads "3:25".

Given a non-negative integer n which represents the number of LEDs that are currently on, return all possible times the watch could represent.

Example:

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

Note:

• The order of output does not matter.
• The hour must not contain a leading zero, for example "01:00" is not valid, it should be "1:00".
• The minute must be consist of two digits and may contain a leading zero, for example "10:2" is not valid, it should be "10:02".

Tags

• Bit

Thought

Reference: http://bookshadow.com/weblog/2016/09/18/leetcode-binary-watch/

The total number of situations for 10 LEDs is 1024. Thus, we can loop from 0 until 1024 and count the number of 1 in the binary number and check whether it is valid.

Code

class Solution(object):
    def readBinaryWatch(self, num):
        """
        :type num: int
        :rtype: List[str]
        """
        ans = []
        for x in range(1024):
            if bin(x).count('1') == num:
                h, m = x >> 6, x & 0x3F
                if h < 12 and m < 60:
                    ans.append('%d:%02d' % (h, m))
        return ans