双向链表是另一种形式的链表，与单链表不同，双向链表的节点具有两个指针，分别指向节点的直接前驱和直接后继。因此双向链表可以双向遍历。
老师代码如下:
#include <stdio.h>
#include <malloc.h>
/**
* Double linked list of integers. The key is char.
*/
typedef struct DoubleLinkedNode{
char data;
struct DoubleLinkedNode *previous;
struct DoubleLinkedNode *next;
} DLNode, *DLNodePtr;
/**
* Initialize the list with a header.
* @return The pointer to the header.
*/
DLNodePtr initLinkList(){
DLNodePtr tempHeader = (DLNodePtr)malloc(sizeof(struct DoubleLinkedNode));
tempHeader->data = '\0';
tempHeader->previous = NULL;
tempHeader->next = NULL;
return tempHeader;
}// Of initLinkList
/**
* Print the list.
* @param paraHeader The header of the list.
*/
void printList(DLNodePtr paraHeader){
DLNodePtr p = paraHeader->next;
while (p != NULL) {
printf("%c", p->data);
p = p->next;
}// Of while
printf("\r\n");
}// Of printList
/**
* Insert an element to the given position.
* @param paraHeader The header of the list.
* @param paraChar The given char.
* @param paraPosition The given position.
*/
void insertElement(DLNodePtr paraHeader, char paraChar, int paraPosition){
DLNodePtr p, q, r;
// Step 1. Search to the position.
p = paraHeader;
for (int i = 0; i < paraPosition; i ++) {
p = p->next;
if (p == NULL) {
printf("The position %d is beyond the scope of the list.", paraPosition);
return;
}// Of if
} // Of for i
// Step 2. Construct a new node.
q = (DLNodePtr)malloc(sizeof(struct DoubleLinkedNode));
q->data = paraChar;
// Step 3. Now link.
r = p->next;
q->next = p->next;
q->previous = p;
p->next = q;
if (r != NULL) {
r->previous = q;
}// Of if
}// Of insertElement
/**
* Delete an element from the list.
* @param paraHeader The header of the list.
* @param paraChar The given char.
*/
void deleteElement(DLNodePtr paraHeader, char paraChar){
DLNodePtr p, q, r;
p = paraHeader;
// Step 1. Locate.
while ((p->next != NULL) && (p->next->data != paraChar)){
p = p->next;
}// Of while
// Step 2. Error check.
if (p->next == NULL) {
printf("The char '%c' does not exist.\r\n", paraChar);
return;
}// Of if
// Step 3. Change links.
q = p->next;
r = q->next;
p->next = r;
if (r != NULL) {
r->previous = p;
}// Of if
// Step 4. Free the space.
free(q);
}// Of deleteElement
/**
* Unit test.
*/
void insertDeleteTest(){
// Step 1. Initialize an empty list.
DLNodePtr tempList = initLinkList();
printList(tempList);
// Step 2. Add some characters.
insertElement(tempList, 'H', 0);
insertElement(tempList, 'e', 1);
insertElement(tempList, 'l', 2);
insertElement(tempList, 'l', 3);
insertElement(tempList, 'o', 4);
insertElement(tempList, '!', 5);
printList(tempList);
// Step 3. Delete some characters (the first occurrence).
deleteElement(tempList, 'e');
deleteElement(tempList, 'a');
deleteElement(tempList, 'o');
printList(tempList);
// Step 4. Insert to a given position.
insertElement(tempList, 'o', 1);
printList(tempList);
}// Of appendInsertDeleteTest
/**
* Address test: beyond the book.
*/
void basicAddressTest(){
DLNode tempNode1, tempNode2;
tempNode1.data = 4;
tempNode1.next = NULL;
tempNode2.data = 6;
tempNode2.next = NULL;
printf("The first node: %d, %d, %d\r\n",
&tempNode1, &tempNode1.data, &tempNode1.next);
printf("The second node: %d, %d, %d\r\n",
&tempNode2, &tempNode2.data, &tempNode2.next);
tempNode1.next = &tempNode2;
}// Of basicAddressTest
/**
* The entrance.
*/
void main(){
insertDeleteTest();
basicAddressTest();
}// Of main
