convert-bst-to-greater-tree.java

// 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]
//
//
// Example 2:
//
//
// Input: root = [0,null,1]
// Output: [1,null,1]
//
//
//  
// Constraints:
//
//
// 	The number of nodes in the tree is in the range [0, 104].
// 	-104 <= Node.val <= 104
// 	All the values in the tree are unique.
// 	root is guaranteed to be a valid binary search tree.
//
//
//  
// Note: This question is the same as 1038: https://leetcode.com/problems/binary-search-tree-to-greater-sum-tree/
//


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