Invert Binary Tree.js

/*Invert a binary tree.
     4
   /   \
  2     7
 / \   / \
1   3 6   9
to
     4
   /   \
  7     2
 / \   / \
9   6 3   1*/

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {TreeNode}
 */
var invertTree = function(root) {
    if(root == null)
        return null
    temp = root.right
    root.right = root.left
    root.left = temp
    invertTree(root.left)
    invertTree(root.right)
    return root
};