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Difficulty: Easy

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.

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".

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Backtracking Bit Manipulation
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// solution 1  from 光头哥
  public List<String> readBinaryWatch1(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;        
    }
    
//solution 2: using backtracking
      public List<String> readBinaryWatch(int num) {
        List<String> res = new ArrayList<>();
        int[] nums1 = new int[]{8, 4, 2, 1}, nums2 = new int[]{32, 16, 8, 4, 2, 1};
        for(int i = 0; i <= num; i++) {
            List<Integer> list1 = generateDigit(nums1, i);
            List<Integer> list2 = generateDigit(nums2, num - i);
            for(int num1: list1) {
                if(num1 >= 12) continue;
                for(int num2: list2) {
                    if(num2 >= 60) continue;
                    res.add(num1 + ":" + (num2 < 10 ? "0" + num2 : num2));
                }
            }
        }
        return res;
    }

    private List<Integer> generateDigit(int[] nums, int count) {
        List<Integer> res = new ArrayList<>();
        generateDigitHelper(nums, count, 0, 0, res);
        return res;
    }

    private void generateDigitHelper(int[] nums, int count, int pos, int sum, List<Integer> res) {
        if(count == 0) {
            res.add(sum);
            return;
        }
        
        for(int i = pos; i < nums.length; i++) {
            generateDigitHelper(nums, count - 1, i + 1, sum + nums[i], res);    
        }
    }