C语言实现双向循环链表

双向链表循环，完成插入删除搜素打印基本功能。

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

typedef int elemType;

typedef struct listnode{
	elemType info;
	struct listnode * previous;
	struct listnode * next;
}listnode;

listnode * linkListInit(listnode * head){
	head = NULL;
	return head;
}

listnode * addNode(listnode *head,elemType value){
	listnode *ptr = (listnode *)malloc(sizeof(listnode));
	ptr->info = value;
	if( NULL == head){
        ptr->previous = NULL;
        ptr->next = NULL;
        head = ptr;
        return head;
	}
	if( head->next == NULL){
        head->next = ptr;
        head->previous = ptr;
        ptr->previous = head;
        ptr->next = head;
        return head;
	}
	ptr->previous = head->previous;
	head->previous->next = ptr;
	head->previous = ptr;
	ptr->next = head;
	return head;
}

listnode * findNode(listnode * head,elemType value){
	listnode *ptr = head;
	if(NULL == head)//0 elem
        return NULL;
	else if(NULL == ptr->next){ //1 elem
        if(value == ptr->info)
            return ptr;
        else 
            return NULL;
	}
	do{
        if(value == ptr->info)
            return ptr;
        ptr = ptr->next;

	}while(ptr != head);
	return NULL;
}

listnode * delNode(listnode * head,elemType value){
	listnode *elemPtr = findNode(head,value);
	listnode *pre,*next;
	if(NULL == elemPtr)
        printf("Cannot find value: %d in list.\n",value);
	else{
        if(head->next == NULL){
            head = NULL;
            free(elemPtr);
        }
        else if(head->next == head->previous){
            if(head == elemPtr){
                head = head->next;
                head->next = NULL;
                head->previous = NULL;
                free(elemPtr);
            }
            else{
                head->next = NULL;
                head->previous = NULL;
                free(elemPtr);
            }
        }
        else{
            pre = elemPtr->previous;
            next = elemPtr->next;
            elemPtr->previous->next = next;
            elemPtr->next->previous = pre;
            if(elemPtr == head)
                head = next;
            free(elemPtr);
        }
	}
	return head;
}


void printList(listnode *head){
	if(NULL == head){
        printf("No elements in list.\n");
        return;
	}
	listnode *ptr = head;
	printf("printf from head to rear:\n");
	do{
        printf("%d -> ",ptr->info);
        ptr = ptr->next;
	}while(ptr != head && ptr != NULL);
	printf("\n");
	printf("printf from rear to head:\n");
	ptr = head;
	if(NULL == ptr->previous){
        printf("%d -> \n",ptr->info);
        return ;
	}
	do{
        ptr = ptr->previous;
        printf("%d -> ",ptr->info);
	}while(ptr != head);
	printf("\n");

}

int main(){
	listnode *head;
	int i;
	head=linkListInit(head);
	for(i=0;i<10;i++){
        head=addNode(head,i);
	}
	printList(head);
	head = delNode(head,2);
	printList(head);
	head = delNode(head,0);
	printList(head);
	return 0;
}

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