二叉树的最大深度

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

def solve():
    def maxDepth(root):
        if not root:
            return 0
        return 1 + max(maxDepth(root.left), maxDepth(root.right))
    root = TreeNode(3, TreeNode(9), TreeNode(20, TreeNode(15), TreeNode(7)))
    print(maxDepth(root))