How to mirror a binary tree

Easy, but you have to be prepared for anything :) Mirroring means switching the pointers around, inside of the tree, so that the resulting tree looks like it's mirror copy of the original tree. Here's an example.
So the tree:

   1:             10

   2:            /   \

   3:           7     25

   4:         /   \

   5:        2     8

becomes:

   1:             10

   2:            /   \

   3:           25    7

   4:               /   \

   5:              8     2

The solution has to be O(n) because it has to change all the pointers from all the nodes, so it can't be done faster.

   1:  void mirror(struct node* head) {

   2:      if (head == 0)

   3:          return;

   4:      node* temp = head->left;

   5:      head->left = head->right;

   6:      head->right = temp; // switch left child with right child

   7:      mirror(head->left); // then repeat the process with each child

   8:      mirror(head->right);

   9:  }

This can be easily expanded for generic trees with more than two children, the basic idea remains the same.