algorithm - 间接在这段代码中是如何工作的？

Question

我正在阅读Merging two sorted linked list的答案。编码：

#define SWAP_PTRS(a, b) do { void *t = (a); (a) = (b); (b) = t; } while (0)

Node* MergeLists(Node* list1, Node* list2) 
{
  Node *list = NULL, **pnext = &list;

  if (list2 == NULL)
    return list1;

  while (list1 != NULL)
  {
    if (list1->data > list2->data)
      SWAP_PTRS(list1, list2);

   // What does the following piece of code do ?
    *pnext = list1;  // ??
    pnext = &list1->next; // ??
    list1 = *pnext;     // ?? 
  }

  *pnext = list2;
  return list;
}

我无法理解双指针在这里是如何工作的，值是如何改变的？

Answer 1

pnext is a pointer to a pointer of Node and is meant to hold the address of the next field of the last node

so the first line sets the pointer to the next node (either list or the previous node->next)

the second line sets pnext to the next field of the current node

the third line advances list1 after just dealing with the head of it using the just assigned pnext for a micro-optimization that avoids dereferencing list1 again

you can also write it in terms of node->next:

#define SWAP_PTRS(a, b) do { void *t = (a); (a) = (b); (b) = t; } while (0)

Node* MergeLists(Node* list1, Node* list2) 
{
  Node *list = NULL, *node ;

  if (list2 == NULL)
    return list1;

  if (list1->data > list2->data)
    SWAP_PTRS(list1, list2);

  node=list=list1;

  list1=list1->next;

  while (list1 != NULL)
  {
    if (list1->data > list2->data)
      SWAP_PTRS(list1, list2);

    node->next = list1;  

    node = list1->next; 
    list1 = list1->next;     
  }

  node->next = list2;
  return list;
}