13.Convert BST to Greater Tree（将树转为更大树）

Posted 2021-02-16 yjxyy

Level：

??Easy

题目描述：

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

思路分析：

??中序遍历是先访问左子树然后根节点，然后右子树，这道题我们可以修改一下中序遍历的顺序，先访问右子树，然后根节点，然后左子树，在访问的过程中我们更新节点的值，最后得到题目的结果。

代码：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    int sum=0;
    public TreeNode convertBST(TreeNode root) {
        if(root==null)
            return null;
        if(root!=null){
            convertBST(root.right);//先访问右子树
            root.val=root.val+sum; //更新节点的值
            sum=root.val;
            convertBST(root.left);
        }
        return root;
    }
}