LeetCode: Max depth of a tree

Given a binary tree, find its maximum depth.

The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

Note: A leaf is a node with no children.

Example:

Given binary tree [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

return its depth = 3.

/**
 * 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;
 *     }
 * }
 */
/*
* ITERATIVE SOLUTION
*/
class Solution {
    public int maxDepth(TreeNode node) {
		 if(node == null) {
			 return 0;
		 }
		 
		 Queue<TreeNode> queue = new LinkedList();
		 queue.add(node); // add root node to the queue
		 int depth = 0 ;
		 
		 while(!queue.isEmpty()) {
			 int size = queue.size(); // ıterate over all added sub nodes
			 for(int i = 0 ; i < size ; i++) { // add all sub nodes into queue
				 TreeNode currentNode = queue.remove();
				 if(currentNode.left != null) {
					 queue.add(currentNode.left);
				 }
				 if(currentNode.right != null) {
					 queue.add(currentNode.right);
				 }
			 }
			 depth ++; // increase depth by one after adding all sub nodes to queue
		 }
		 return depth;
    }
    

}

// RECURSIVE SOLUTION    O(N), O(N)
public int depth(TreeNode root, int sum) {
        if (root == null) {
            return sum;
        }
        return Math.max(depth(root.left,sum+1), depth(root.right,sum+1));
    }