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".

一刷
题解：
最简单的做法：00:00-11:59遍历，然后用Integer.bitCount来计算满足要求的时间

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) + Integer.bitCount(m)) == num){
                    if(m<10){
                        times.add(String.format("%d:0%d",h,m));
                    }else{
                        times.add(String.format("%d:%d",h,m));
                    }
                    
                }
            }
        }
        return times;
    }
}