Largest Time for Given Digits LeetCode solution in Java

1
class Solution {
2
  public String largestTimeFromDigits(int[] arr) {
3
    int[] count = new int[10];
4
    for (int num : arr) {
5
      count[num]++;
6
    }
7
    StringBuilder sb = backTrack(count, new StringBuilder());
8
    if (sb.length() == 4) sb.insert(2, ':');
9
    return sb.toString();
10
  }
11

12
  private StringBuilder backTrack(int[] count, StringBuilder sb) {
13
    int size = sb.length();
14
    int start = 0;
15
    if (size == 0) {
16
      start = 2;
17
    }
18
    if (size == 1) {
19
      start = sb.charAt(0) == '2' ? 3 : 9;
20
    }
21
    if (size == 2) {
22
      start = 5;
23
    }
24
    if (size == 3) {
25
      start = 9;
26
    }
27
    for (int i = start; i >= 0; i--) {
28
      if (count[i] == 0) continue;
29
      count[i]--;
30
      sb.append(i);
31
      backTrack(count, sb);
32
      if (sb.length() == 4) {
33
        return sb;
34
      } else {
35
        count[Character.getNumericValue(sb.charAt(sb.length() - 1))]++;
36
        sb.deleteCharAt(sb.length() - 1);
37
      }
38
    }
39
    return sb;
40
  }
41
}