题目:找出二叉树的最大深度

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 1

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

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

• 解法

var maxDepth = function(root) {
    if(!root) return 0
    return 1 + Math.max(maxDepth(root.left),maxDepth(root.right))
};