/**
* Definition for a binary tree node.
* function TreeNode(val, left, right) {
* this.val = (val===undefined ? 0 : val)
* this.left = (left===undefined ? null : left)
* this.right = (right===undefined ? null : right)
* }
*/
/**
* @param {TreeNode} root
* @return {number}
*/
var maxDepth = function(root) {
return getDepth(root);
};
var getDepth = function(root) {
if(root === null)
return 0;
let left = getDepth(root.left);
let right = getDepth(root.right);
if(left > right)
return left + 1;
else
return right + 1;
}