538. Convert BST to Greater Tree

 

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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:
              5
            /   
           2     13

Output: The root of a Greater Tree like this:
             18
            /   
          20     13

class Solution {
    int sum = 0;
    public TreeNode convertBST(TreeNode root) {
        convert(root);
        return root;
    }
    public void convert(TreeNode root){
        if(root==null) return;
        convert(root.right);
        root.val+=sum;
        sum = root.val;
        convert(root.left);
    } 
}