Convert BST to Greater Tree

反中序遍历就可以解决了

class Solution(object):
    def convertBST(self, root):
        """
        :type cur: TreeNode
        :rtype: TreeNode
        """
        tmp = 0
        stack = []
        cur = root
        while cur or stack:
            if cur:
                stack.append(cur)
                cur = cur.right
            else:
                cur = stack.pop()
                cur.val += tmp
                tmp = cur.val
                cur = cur.left
        return root