401. Binary Watch*

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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".

    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;    }

总结：列举法

h*64是为了左移六位

Integer.bitCount是计算num中二进制补码所含1的数量

%d%02d，不一定要列举所有情况，可以选择格式化输出

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