Convert BST to Greater 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 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.

Note: This question is the same as 1038: https://leetcode.com/problems/binary-search-tree-to-greater-sum-tree/

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]

Example 3: Input: root = [1,0,2] Output: [3,3,2]

Example 4: Input: root = [3,2,4,1] Output: [7,9,4,10] 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.

Solution:

class Solution {
public TreeNode convertBST(TreeNode root) {
        Stack<TreeNode> stack = new Stack<>();
        TreeNode node = root;
        int sum=0;
        while(!stack.isEmpty() || node!=null)
        {
            while(node!=null)
        {
            stack.add(node);
            node=node.right;
        }
        
        node = stack.pop();
        sum+=node.val;
        node.val=sum;
        
        node = node.left;
        
        }
        
        return root;
    }
}