如何找到树的最大深度？

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

def max_depth(root):
    if root is None:
        return 0
    else:
        left_depth = max_depth(root.left)
        right_depth = max_depth(root.right)
        return max(left_depth, right_depth) + 1

# 示例用法
root = TreeNode(3)
root.left = TreeNode(9)
root.right = TreeNode(20)
root.right.left = TreeNode(15)
root.right.right = TreeNode(7)

print("树的最大深度是:", max_depth(root))  # 输出: 3