LC 226. Invert Binary Tree

 1/**
 2 * Definition for a binary tree node.
 3 * function TreeNode(val, left, right) {
 4 *     this.val = (val===undefined ? 0 : val)
 5 *     this.left = (left===undefined ? null : left)
 6 *     this.right = (right===undefined ? null : right)
 7 * }
 8 */
 9/**
10 * @param {TreeNode} root
11 * @return {TreeNode}
12 */
13var invertTree = function (root) {
14  if (root === null) return null
15  const left = invertTree(root.left)
16  const right = invertTree(root.right)
17  root.left = right
18  root.right = left
19  return root
20};