/**
 * 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) {
    let deep = 0;
    let depth = function(root,d){
        if(!root) {
            return;
        }else{
            d++;
        }
        if(!root.left && !root.right){
            deep = Math.max(deep,d);
        }
        depth(root.left,d);
        depth(root.right,d);
    }
    depth(root,deep);
    return deep;
};