Leetcode学习笔记：#1038. Binary Search Tree to Greater Sum Tree

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于 2019-05-08 17:01:03 发布

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该博客是Leetcode #1038题学习笔记，题目要求修改二叉搜索树，使每个节点值等于原树中大于或等于该节点值的和。介绍了二叉搜索树约束条件，实现思路为反中序遍历，先算右子树和，再遍历左子树并返回root。

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Leetcode学习笔记：#1038. Binary Search Tree to Greater Sum Tree

Given the root of a binary search tree with distinct values, modify it so that every node has a new value equal to the sum of the values of the original tree that are greater than or equal to node.val.

As a reminder, a binary search tree is a tree that satisfies these constraints:

The left subtree of a node contains only nodes with keys less than the node’s key.
The right subtree of a node contains only nodes with keys greater than the node’s key.
Both the left and right subtrees must also be binary search trees.

实现：

int pre = 0;
public TreeNode bstToGst(TreeNode root){
	if(root.right != null)
		bstToGst(root.right);
	pre = root.val = pre + root.val;
	if(root.left != null)
		bstToGst(root.left);
	return root;
}

思路：
反中序遍历。先遍历右子树，把右子树的和先算出来，因为右子树永远比左子树大，所以再遍历左子树，返回root；