CODE

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

public class Solution {

  int maxDepth = 0;
  int depth = 0;

  public int MaxDepth(TreeNode root) {
    if (root == null)
    {
      if (depth > maxDepth)
        maxDepth = depth;
      return depth;
    }
    depth++;
    MaxDepth(root.left);
    MaxDepth(root.right);
    depth--;
    
    return maxDepth;

  }
}