### 104. Maximum Depth of Binary Tree

Given a binary tree, find its maximum depth.

The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

Note: A leaf is a node with no children.

Example:

Given binary tree [3,9,20,null,null,15,7],

3   / \  9  20    /  \   15   7
return its depth = 3.

### Analyze

Way1: recursive

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

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