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


def maxDepth(root):
    """
    :type root: TreeNode
    :rtype: int
    """
    # 如果树为空，深度为0
    if not root:
        return 0

    # 递归计算左子树的最大深度
    left_depth = maxDepth(root.left)

    # 递归计算右子树的最大深度
    right_depth = maxDepth(root.right)

    # 当前树的最大深度是左右子树最大深度的较大值加1（根节点）
    return max(left_depth, right_depth) + 1


# 示例 1
root1 = TreeNode(3)
root1.left = TreeNode(9)
root1.right = TreeNode(20)
root1.right.left = TreeNode(15)
# root1.right.right = TreeNode(7)

# 示例 2
root2 = TreeNode(1)
root2.right = TreeNode(2)

# 调用函数并打印结果
print(maxDepth(root1))  # 应输出 3
print(maxDepth(root2))  # 应输出 2