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"]

还算有趣的题目，而且这手表不错，不知道哪里有。

一开始看，就感觉等同于一道题目，求一个范围内的二进制数中1的个数限定的数

这个题目的解法我一直没很好的掌握，上蠢办法，范围内全部遍历

public class Solution {
    public List<String> readBinaryWatch(int num) {
        List<String> answer = new ArrayList<>();
        for(int i=0;i<1024;++i){
            int curNum=0;
            int cur=i;
            while(cur!=0){
                curNum+=cur&1;
                cur>>=1;
            }
            if(curNum==num && i%64<=59 && i/64<=11){
                answer.add(String.format("%d:%02d",i/64,i%64).toString());
            }
        }
        return answer;
    }
}