/**
 * 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 {
    public int maxDepth(TreeNode root) {
        int[] answer = new int[1];
        if(root == null) return answer[0];
        maxDepth(root, 0, answer);
        return answer[0];
    }
    public void maxDepth(TreeNode node, int currentDepth, int[] answer){
        currentDepth += 1;
        if(node.left != null) maxDepth(node.left, currentDepth, answer);
        if(node.right != null) maxDepth(node.right, currentDepth, answer);
        if(node.right == null && node.left == null) answer[0] = Math.max(answer[0], currentDepth);
        return;
    }
}