Increasing Order Search Tree

Problem statement

Given the root of a binary search tree, rearrange the tree in in-order so that the leftmost node in the tree is now the root of the tree, and every node has no left child and only one right child.

Example 1:

Input: root = [5,3,6,2,4,null,8,1,null,null,null,7,9]
Output: [1,null,2,null,3,null,4,null,5,null,6,null,7,null,8,null,9]

Example 2:

Input: root = [5,1,7]
Output: [1,null,5,null,7]

Constraints:

• The number of nodes in the given tree will be in the range [1, 100].
• 0 <= Node.val <= 1000

My solution

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {TreeNode}
 */
var increasingBST = function(root) {
    let newTree = null;
    let prev = null;
    
    function preOrder(node) {
        if (!node) {
            return;
        }
        
        if (node.left) {
            preOrder(node.left)
        }
        
        if (!newTree) {
            newTree = new TreeNode(node.val);
            prev = newTree
        } else {
            prev.right = new TreeNode(node.val);
            prev = prev.right;
        }
        
        if (node.right) {
            preOrder(node.right)
        }
    }
    
    preOrder(root)
    
    return newTree;
};