Question: "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.
Input: root = [3,9,20,null,null,15,7] Output: 3 "
//javascript
var maxDepth = function(root) {
if(root == null) return 0;
const left = maxDepth(root.left);
const right = maxDepth(root.right);
return Math.max(left, right) + 1;
}