538. Convert BST to Greater Tree 二叉搜索树转换为更大树

Given a Binary Search Tree (BST), convert it to a Greater Tree such that every key of the original BST is changed to the original key plus sum of all keys greater than the original key in BST.

Example:

Input: The root of a Binary Search Tree like this:
              5
            /   
           2     13

Output: The root of a Greater Tree like this:
             18
            /   
          20     13

题意：给出二叉搜索树，每一个节点累加比自己更大的值

解法：根据二叉搜索树的特性可知，按照 右->根->左的顺序遍历节点

# 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 convertBST(self, root):
        """
        :type root: TreeNode
        :rtype: TreeNode
        """
        self.helper(root)
        return root
        
        
    sum = 0
    def helper(self, node):
        if(node):
            self.helper(node.right)
            node.val += self.sum
            self.sum = node.val
            self.helper(node.left)

来自为知笔记(Wiz)