ConvertBSTtoGreaterTree.js

var convertBST = (root)=>{
    let sum = 0; 
    function update(root) {
        if (!root) return;      
        update(root.right);        
        root.val+=sum;
        sum=root.val;        
        update(root.left);
    }    
    update(root);  
    return root;};

/*

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

*/