Binary Tree Upside Down.js

/**
 Given a binary tree where all the right nodes are either leaf nodes with a sibling (a left node that shares the same parent node) or empty, flip it upside down and turn it into a tree where the original right nodes turned into left leaf nodes. Return the new root.
For example:
Given a binary tree {1,2,3,4,5},

    1
   / \
  2   3
 / \
4   5

return the root of the binary tree [4,5,2,#,#,3,1].

   4
  / \
 5   2
    / \
   3   1  

confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.
*/
/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {TreeNode}
 */
var upsideDownBinaryTree = function(root) {
    if (!root || (!root.left && !root.right)) {
        return root;
    }
    
    let newRoot = upsideDownBinaryTree(root.left);
    
    root.left.left = root.right;
    root.left.right = root;
    
    root.left = null;
    root.right = null;
    
    return newRoot;
};