Binary Search Tree to Greater Sum Tree

Given the root of 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 the sum of all keys greater than the original key in BST.

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.

Example 1:

Input: root = [4,1,6,0,2,5,7,null,null,null,3,null,null,null,8]
Output: [30,36,21,36,35,26,15,null,null,null,33,null,null,null,8]

思路：reverse inorder traverse，也就是inorder: left, current, right;

现在需要right, current, left, 然后value进行累加。逆着写inorder就可以了。

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public TreeNode bstToGst(TreeNode root) {
        Stack<TreeNode> stack = new Stack<>();
        pushRight(root, stack);
        TreeNode oldroot = root;
        
        Integer preval = null;
        while(!stack.isEmpty()) {
            TreeNode node = stack.pop();
            if(preval == null) {
                preval = node.val;
            } else {
                node.val = node.val + preval;
                preval = node.val;
            }
            if(node.left != null) {
                pushRight(node.left, stack);
            }
        }
        return oldroot;
    }
    
    public void pushRight(TreeNode node, Stack<TreeNode> stack) {
        while(node != null) {
            stack.push(node);
            node = node.right;
        }
    }
}