Leetcode 104 - Maximum Depth of Binary Tree

Given the root of a binary tree, return its maximum depth.

A binary tree’s maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

Example:

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Input: root = [3,9,20,null,null,15,7]
Output: 3
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/**
* 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) {
if (!root) return 0;

return 1 + Math.max(maxDepth(root.left), maxDepth(root.right));
};