897. Increasing Order Search Tree

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def increasingBST(self, root):
        """
        :type root: TreeNode
        :rtype: TreeNode
        """

        #if root is None: return root

        def helper(node):
            if node is None: return None, None
            if node.left is None and node.right is None:
                return node, node
            left_root, left_end = None, None
            right_root, right_end = None, None
            if node.left:
                left_root, left_end = helper(node.left)
            if node.right:
                right_root, right_end = helper(node.right)

            first, end = None, None

            if left_root:
                first = left_root
                # left_end = node
                left_end.right = node
            else:
                first = node

            node.left = None

            if right_root:
                end = right_end
                node.right = right_root
            else:
                end = node


            return first, end

        return helper(root)[0]

在连接左边子树和当前结点的时候出错了。

                # left_end = node
                left_end.right = node