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:

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