Invert a binary Tree

Invert a binary tree.

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

to

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

Solution:
/**
 * 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) {
        TreeNode* temp = NULL;
        if ((root == NULL))
            return root;
        temp = root -> left;
        root -> left = invertTree(root -> right);
        root -> right = invertTree(temp);
        return root;
    }
    // Iterative
    // BFS with queue.
};