10. [2%, 2%] The following data structure represents a binary tree, and it contains a function named "unknown".

struct node{
    int data;
    struct node *left, *right;
};

void unknown(struct node *p) {
    struct node *q;
    if (p->left != NULL) unknown(p->left);
    if (p->right != NULL) unknown(p->right);
    q = p->left;
    p->left = p->right;
    p->right = q;
}