struct node First, Last; 
 bst2ll(root, &First, &Last); 
 // at this point, First.right points to the first node in the linked list,  
 // while Last.left points to the last. 
   
 void bst2ll(struct node *root, struct node *first, struct node *last) 
 { 
     struct node *above, *cur, *below; 
  
     first->left = NULL; 
     first->right = last; 
     last->left = first; 
     last->right = NULL; 
  
     above = NULL; 
     cur = root; 
  
     /* Try to go down to the left */ 
  down_left: 
     if (cur == NULL) return; 
     below = cur->left; 
     cur->left = above;  // save how to get back up 
     if (below) { 
    above = cur; 
    cur = below; 
    goto down_left; 
     } 
  
  down_right: 
     above = cur->left; // recall how to get back up 
     below = cur->right;     // save how to go down to the right 
  
     /* Add cur to the list */ 
     cur->left = last->left; 
     last->left = cur; 
     cur->left->right = cur; 
     cur->right = last; 
  
     /* Try to go down to the right */ 
     /* We leave "above" alone because we don't want to return 
   to cur anymore -- it's been removed from the tree and 
   placed in the list.  Instead, we want to return to the 
   nearest ancestor of cur whose right child we haven't 
   yet checked.  This is "above". */ 
     if (below) { 
    cur = below; 
    goto down_left; 
     } 
  
     /* Go back up */ 
     cur = above; 
     goto down_right; 
 }