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) {
        queue <TreeNode* > tree;
        if(root==NULL)
          return NULL;
        tree.push(root);
        while(!tree.empty())
        {
            TreeNode* tmpnode = tree.front();
            tree.pop();
            TreeNode* left=tmpnode->left;
            tmpnode->left=tmpnode->right;
            tmpnode->right=left;
            if(tmpnode->left)
              tree.push(tmpnode->left);
            if(tmpnode->right)
              tree.push(tmpnode->right);
        }
        return root;
    }
};