538. Convert BST to Greater Tree

Description

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:

BST

Output: The root of a Greater Tree like this:

Greater tree

Solution

DFS, time O(n), space O(1)

对于每个节点，node.val += sum(all nodes val on the right)

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode convertBST(TreeNode root) {
        convertBSTHelper(root, 0);
        return root;
    }
    
    public int convertBSTHelper(TreeNode root, int sum) {
        if (root == null) {
            return sum;
        }
        
        root.val += convertBSTHelper(root.right, sum);
        return convertBSTHelper(root.left, root.val);
    }
}