給定一棵二叉樹，返回它的最大深度

最大深度是指樹中最長路徑所擁有的結點數量

注意：葉子節點沒有子節點

例如：

給定二叉樹[3,9,20,null,null,15,7]

  3
   / \
  9  20
    /  \
   15   7

它的返回值為3

def maxDepth(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        if not root:
            return 0
        curLevelNodeList = [root]
        length = 0
        while curLevelNodeList:
            tempNodeList = [] 
            for node in curLevelNodeList:
                if node.left is not None:
                    tempNodeList.append(node.left)
                if node.right is not None:
                    tempNodeList.append(node.right)
            curLevelNodeList = tempNodeList
            length += 1
        return length
 

2：遞迴，比較每一條路徑的長短

def maxDepth(self, root):
        """
        :type root: TreeNode
        :rtype: int
        """
        if not root:
            return 0
        left = self.maxDepth(root.left)+1
        right = self.maxDepth(root.right)+1
        
        return max(left, right)