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".
题目含义：就是用二进制的方式实现的手表，Led灯来表示二进制，亮着表示置位1，不亮表示0。4个位表示小时，6个位表示分钟。题目要求给定亮着的Led灯个数，求出所有可以表示的时间列表。
1 public List<String> readBinaryWatch(int num) { 2 List<String> times = new ArrayList<>(); 3 for (int h = 0; h < 12; h++) 4 for (int m = 0; m < 60; m++) 5 if (Integer.bitCount(h) + Integer.bitCount(m) == num) 6 times.add(String.format("%d:%02d", h, m)); 7 return times; 8 }