Invert Binary Tree

Invert a binary tree.

     4
   /   
  2     7
 /    / 
1   3 6   9

     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.

 1 /**
 2  * Definition for a binary tree node.
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 public:
12     TreeNode* invertTree(TreeNode* root) {
13         queue <TreeNode* > tree;
14         if(root==NULL)
15           return NULL;
16         tree.push(root);
17         while(!tree.empty())
18         {
19             TreeNode* tmpnode = tree.front();
20             tree.pop();
21             TreeNode* left=tmpnode->left;
22             tmpnode->left=tmpnode->right;
23             tmpnode->right=left;
24             if(tmpnode->left)
25               tree.push(tmpnode->left);
26             if(tmpnode->right)
27               tree.push(tmpnode->right);
28         }
29         return root;
30     }
31 };