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LeetCode 401. Binary Watch

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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".

Explanation

利用位运算将状态拆分为小时和分钟。 枚举小时h和分钟m，然后判断二进制1的个数是否等于num

Code

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class Solution(object):
    def readBinaryWatch(self, num):
        """
        :type num: int
        :rtype: List[str]
        """
        result = []
        for h in range(12):
            for m in range(60):
                if (bin(h) + bin(m)).count('1') == num:
                    result.append('%d:%02d' % (h, m))
        return result