Greater Than Tree Solution

greaterThanTree.js

let sum = 0 // sum will be added to all 'less than' nodes
const convertBST = (root) => { 
    if (root == null) return; 
    convertBST(root.right); // traverse right
    sum += root.val; // add root value to 'global' sum
    root.val = sum; // new value becomes sum of all values that have already been seen
    convertBST(root.left) // traverse left
    return root;
}