1. 二叉树的深度
输入一棵二叉树的根节点，求该树的深度。从根节点到叶节点依次经过的节点（含根、叶节点）形成树的一条路径，最长路径的长度为树的深度。
方法一：

class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right

def maxDepth(root):
    if root is None:
        return 0
    return max(maxDepth(root.left), maxDepth(root.right)) + 1

方法二：

class TreeNode:
    def __init__(self, val=0, left=None, right=None):
        self.val = val
        self.left = left
        self.right = right

def maxDepth(root):
    if root is None:
        return 0
    depth = 0
    queue = [root]
    while queue:
        depth += 1
        level_size = len(queue)
        for _ in range(level_size):
            node = queue.pop(0)
            if node.left:
                queue.append(node.left)
            if node.right:
                queue.append(node.right)
    return depth