Question

我是C的新手，一直致力于手工编写链表。现在，我正在研究一种从列表前面删除元素的方法。它写着：

   void remove_from_front(List *l)
    {
      Node *head = l->first;

      l->first = l->first->next;

      free(head);
    }

我确信这是显而易见的，但我没有得到它。以下是我的问题的一些相关数据：

之前的研究。我查看了多个SO帖子，包括this one，但都使用了我似乎无法分开的不同实现。整个互联网上都有这些代码的例子，但我不明白它们是如何工作的，而是宁愿了解我的代码出错了。

相关代码段。以下是我程序中的其余代码：

#include <stdio.h>
#include <stdlib.h>

typedef struct Node{
  void *element;
  struct Node *next;
  struct Node *prev;
} Node;

typedef struct List{
  struct Node *first;
  struct Node *last;
} List;

void add_to_front(List *l, void *toAdd)
{
  Node *n = (Node *)calloc(1, sizeof(Node));
  n->element = toAdd;
  n->next = l->first;
  n->prev = NULL;

  if(l->first != NULL)
    {
      l->first->prev = n;
    }
    l->first = n;
}


void add_to_back(List *l, void *toAdd)
{
  Node *n = (Node *)calloc(1, sizeof(Node));
  n->element = toAdd;
  n->next = NULL;
  n->prev = l->last;

  if(l->last != NULL)
    {
      l->last->next = n;
    }
    l->last = n;
}

void remove_from_front(List *l)
{
  Node *head = l->first;

  l->first = l->first->next;

  free(head);
}

int main(int argc, char *argv[])
{
  List *l = calloc(1, sizeof(List));
  l->first = NULL;
  l->last = NULL;

  add_to_front(l, (void *)'a');
  printf("First element: %c\n", l->first->element);
  add_to_back(l, (void *)'b');
  printf("Last element: %c\n", l->last->element);
  remove_from_front(l);
  printf("New first element: %c\n", l->first->element);
  return 0;
}

调试器输出。这是我运行此代码时得到的调试器输出。

Valgrind的：

==9389== Invalid write of size 4
==9389==    at 0x804852E: remove_from_front (in /home/vagrant/plc/linkedList)
==9389==    by 0x80485D3: main (in /home/vagrant/plc/linkedList)
==9389==  Address 0x8 is not stack'd, malloc'd or (recently) free'd
==9389==
==9389==
==9389== Process terminating with default action of signal 11 (SIGSEGV)
==9389==  Access not within mapped region at address 0x8
==9389==    at 0x804852E: remove_from_front (in /home/vagrant/plc/linkedList)
==9389==    by 0x80485D3: main (in /home/vagrant/plc/linkedList)
==9389==  If you believe this happened as a result of a stack
==9389==  overflow in your program's main thread (unlikely but
==9389==  possible), you can try to increase the size of the
==9389==  main thread stack using the --main-stacksize= flag.
==9389==  The main thread stack size used in this run was 8388608.
==9389==

GDB：

Starting program: /home/vagrant/plc/linkedList
First element: a
Last element: b

Program received signal SIGSEGV, Segmentation fault.
0x0804852e in remove_from_front ()

我在没有调试器的情况下运行可执行文件时的输出只是：

First element: a
Last element: b
Segmentation fault

如果可能，我将不胜感激。谢谢！

Answer 1

问题在于，当你在前面添加时，如果它是列表中的第一个元素，你需要设置最后一个指针，让它指向第一个元素，也就是说，当列表有单个时element，first和last应该指向同一个Node。

// in add_to_front
if (l->last == NULL) {
    l->last = l->first;
}
// add to back:
if (l->first == NULL) {
    l->first = l->last;
}
// in remove from front, check if the list is empty:
if (l->first == NULL)
    return;

Answer 2

第二个问题。如果删除最后一个节点，请清除最后一个参考

void remove_from_front(List *l)
{
  Node *head = l->first;

  l->first = l->first->next;

  if (!l->first)
  {
    l->last = 0;
  }

  free(head);
}

C中的链接列表：我为什么要进行分类？

2 个答案: