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:
TreeNode* invertTree(TreeNode* root)
{

if(!root)
return NULL;
if(!root->left&&!root->right)
return root;
TreeNode* y=root->right;
root->right=invertTree(root->left);
root->left=invertTree(y);
return root;

}
};