Increasing Order Search Tree

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]

/**
 * 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 nodes = [];
  const dfs = (root) => {
    if (root === null) {
      return;
    }
    dfs(root.left);
    nodes.push(root.val);
    dfs(root.right);
  };

  dfs(root);

  let treeNode = new TreeNode(0);
  let current = treeNode;
  for (let i = 0; i < nodes.length; i++) {
    current.right = new TreeNode(nodes[i]);
    current = current.right;
  }

  return treeNode.right;
};