Number of Squareful Arrays LeetCode solution in Java

1
class Solution {
2
  int count;
3

4
  public int numSquarefulPerms(int[] nums) {
5
    int n = nums.length;
6
    if (n < 2) return count;
7
    backtrack(nums, n, 0);
8
    return count;
9
  }
10

11
  void backtrack(int[] nums, int n, int start) {
12
    if (start == n) {
13
      count++;
14
    }
15
    Set<Integer> set = new HashSet<>();
16
    for (int i = start; i < n; i++) {
17
      if (set.contains(nums[i])) continue;
18
      swap(nums, start, i);
19
      if (start == 0 || isPerfectSq(nums[start], nums[start - 1])) backtrack(nums, n, start + 1);
20
      swap(nums, start, i);
21
      set.add(nums[i]);
22
    }
23
  }
24

25
  void swap(int[] A, int a, int b) {
26
    int temp = A[a];
27
    A[a] = A[b];
28
    A[b] = temp;
29
  }
30

31
  boolean isPerfectSq(int a, int b) {
32
    int x = a + b;
33
    double sqrt = Math.sqrt(x);
34
    return (sqrt - (int) sqrt) == 0 ? true : false;
35
  }
36
}