401. Binary Watch （二进制手表）

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 class Solution {
    public List<String> readBinaryWatch(int num) {
		List<String> res = new ArrayList<>();
		int[] time = new int[] { 1, 2, 4, 8, 1, 2, 4, 8, 16, 32 };
		backtrack(res, time, num, 0, 0, 0, 0);
		return res;
	}

	public void backtrack(List<String> res,int[] time,int num,int count,int start,int hour,int min){
		if(hour>11||min>59)
			return;
		if(num==count){
			String hh = ""+hour;
			String mm = min<10?"0"+min:""+min;
			res.add(hh+":"+mm);
			return;
		}
		for(int i=start;i<time.length;i++){
			int newHour = i<4?hour+time[i]:hour;
			int newMin = i<4?min:min+time[i];
			backtrack(res,time,num,count+1,i+1,newHour,newMin);
		}
	}
}

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