Invert Binary Tree

/**
* 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 == NULL)
{
return root;
}
TreeNode* tmp = root -> left;
root -> left = invertTree(root -> right);
root -> right = invertTree(tmp);
return root;
}
};