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# Convert BST to Greater Tree

Updated: Mar 25, 2021

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.

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)
{
node=node.right;
}

node = stack.pop();
sum+=node.val;
node.val=sum;

node = node.left;

}

return root;
}
}```