LeetCode 226. Invert Binary Tree

Invert a binary tree.

     4
   /   \
  2     7
 / \   / \
1   3 6   9
to
     4
   /   \
  7     2
 / \   / \
9   6 3   1

Trivia:
This problem was inspired by this original tweet by Max Howell:

Google: 90% of our engineers use the software you wrote (Homebrew), but you can’t invert a binary tree on a whiteboard so fuck off.

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    void invert(TreeNode* root){
         if(root==NULL) return;
         TreeNode* t=root->right;
         root->right=root->left;
         root->left=t;
            invert(root->left);
            invert(root->right);
    }
    TreeNode* invertTree(TreeNode* root) {
        invert(root);
        return root;
    }
};