401. Binary Watch

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:

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

       
       
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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”.

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       class Solution {
       
       
    public List<String> readBinaryWatch(int num) {
       
       
        List<String> times = new ArrayList<>();
       
       
        for (int h = 0; h < 12; h++)
       
       
            for (int m = 0; m < 60; m++)
       
       
                if (Integer.bitCount(h * 64 + m) == num)
       
       
                    times.add(String.format("%d:%02d", h, m));
       
       
        return times;
       
       
    }
       
       
}
      
      
      
      

       
       
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