1530. Number of Good Leaf Nodes Pairs

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution 
{
    int result = 0;

    public int countPairs(TreeNode root, int distance) 
    {
        dfs(root, distance);
        return result;
  }

  

  public int[] dfs(TreeNode root, int distance) 
  {
        int[] d = new int[distance + 1]; // distance --> number of leaf nodes
        if(root == null) return d;
        if(root.left == null && root.right == null) 
        {
            d[0] = 1;
            return d;
        }

        int[] dl = dfs(root.left, distance);
        int[] dr = dfs(root.right, distance);

        for (int i = 0; i < distance; i++)
        {
            for (int j = 0; j < distance; j++)
            {
                if (i + j + 2 <= distance)
                {
                    result += dl[i] * dr[j];

                }
            
            }


        }

        for (int i = 0; i < distance; ++i)
        {
            d[i + 1] = dl[i] + dr[i];
        }

        return d;


    }

}