问题11：双链表的相关操作

主要涉及双链表的建立、测长、打印、插入结点以及删除结点等。

实际上跟单链表的操作差不多，只是要特别注意双链表多了指向前驱结点的指针，在写相关代码的时候要特别小心，别弄错了。

代码如下：

#include <stdio.h>#include <malloc.h>typedef struct student{     int data;   struct student *next;   struct student *pre;}dnode;dnode *creat(){   dnode *head, *p, *s;   int input, cycle = 1;   head = (dnode *)malloc(sizeof(dnode));   p = head;   while(cycle)   {       printf("\nplease enter the data:");   scanf("%d", &input);   if(input != 0)   {      s = (dnode *)malloc(sizeof(dnode));  s->data = input;  printf("\n%d", s->data);  p->next = s;  s->pre = p;  p = s;   }   else      cycle = 0;   }   head = head->next;   head -> pre = NULL;   printf("\nThe value of the first node is: %d", head->data);   p->next = NULL;   return head;}//双链表测长，可以看出代码其实跟前面的单链表测长一样int length(dnode *head){    dnode *p;p = head;int count = 0;while(p){   p = p->next;   count++;}return count;}//打印链表void print(dnode *head){   dnode *p;   int len = length(head);   printf("\nNOW %d node(s) in the List:", len);   p = head;   if(head != NULL)   {      while(p)  {     printf("\n     ￥￥￥￥￥  %d", p->data); p = p->next;  }   }}//双链表删除结点//双链表的删除也别忘了有大致4种情况//一是没有找到，二是在第一个结点，三是在最后一个结点，四是在中间的情况dnode *delete_from_dlist(dnode *head, int num){   if(head == NULL)   {      printf("\n链表为空！！");      return head;   }     dnode *current = head;   while(num != current->data && current->next != NULL)   {       current = current->next;   }   if(num == current->data)   {       if(current == head)   {       head = head->next;   head->pre = NULL;   free(current);   }   else if(current->next == NULL)   {      current->pre->next = NULL;  free(current);   }   else   {       current->pre->next = current->next;   current->next->pre = current->pre;   free(current);   }   }   else   {      printf("\nNODE NOT FOUND!!");   }   return head;}//双链表插入结点dnode *insert_to_list(dnode *head, int num){   dnode *cur, *insert_node;   cur = head;   insert_node = (dnode *)malloc(sizeof(dnode));   insert_node -> data = num;   while(insert_node->data > cur->data && cur->next != NULL)   {      cur = cur->next;   }   if(insert_node->data <= cur->data)   {      if(cur == head)  {     insert_node->next = cur; cur->pre = insert_node; head = insert_node;  }  else  {     insert_node->next = cur; insert_node->pre = cur->pre; cur->pre->next = insert_node; cur->pre = insert_node;  }   }   else   {      cur->next = insert_node;  insert_node->pre = cur;  insert_node->next = NULL;   }      return head;}int main(void){   dnode *List = creat();   print(List);   List = delete_from_dlist(List, 4);   print(List);   List = insert_to_list(List, 5);   print(List);   return 0;}

运行结果如下：