删除BST中的节点

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

def deleteNode(root, key):
    if not root:
        return root

    if key < root.val:
        root.left = deleteNode(root.left, key)
    elif key > root.val:
        root.right = deleteNode(root.right, key)
    else:
        if not root.left:
            return root.right
        elif not root.right:
            return root.left
        else:
            successor = findSuccessor(root.right)
            root.val = successor.val
            root.right = deleteNode(root.right, successor.val)

    return root

def findSuccessor(node):
    while node.left:
        node = node.left
    return node