数据结构——判断单链表是否有环

#include <stdio.h>#include <malloc.h>#include <stdlib.h>#define INSERT_NUM 100#define FAST_POINT_STEP 2typedef int ElemType;typedef struct LNode{    ElemType data;    struct LNode *next;}LinkList, *pNode;//init the linklistvoid initList(pNode *head){    (*head) = (LinkList*)malloc(sizeof(LinkList));    (*head)->next = NULL;    (*head)->data = 9;}//insert Elementint insertElem(pNode head, ElemType data){    pNode pInsertNode;    pNode pWork;     pWork = head;    pInsertNode = (LinkList*)malloc(sizeof(LinkList));    //memset(pInsertNode, 0, sizeof(LinkList));    if(pInsertNode == NULL){        return 0;    }    pInsertNode->next = NULL;    pInsertNode->data = data;    while(pWork->next != NULL){        pWork = pWork->next;    }    pWork->next = pInsertNode;    return 1;}//print node data one by onevoid printList(pNode head){    printf("in the function printList\n");    if(head == NULL){        printf("List is empty----\n");    }    pNode p = head->next;    do{        printf("%d", p->data);        printf("--->");        p = p->next;    }while(p != NULL);    }//make the LinkList loopvoid makeLoop(pNode head){    pNode p = head;    while(p->next != NULL ){        p = p->next;    }    p->next = head->next;}//tell if is a loop list?int isListLoop(pNode head){    int i;    pNode slow = head;    pNode fast = head;    while(fast){        for(i = 0 ; i < FAST_POINT_STEP ; i++){            fast = fast->next;            if(fast == slow)                return 1;            else if(fast == NULL)                return 0;        }        slow = slow->next;    }    return 0;}void main(){    int i;    pNode head;    initList(&head);        for(i = 0 ; i < INSERT_NUM ; i++){        insertElem(head, i);    }    makeLoop(head);    //printList(head);    if(isListLoop(head))        printf("is a loop list \n");    else        printf("is not a loop list\n");}

        之前在网上看到有人说，判断一个链表是否有环，还可以使用两个for循环嵌套的方法，当外层循环步进一个节点时，内层循环就遍历外层循环节点之后的所有节点，然后比较内外循环的两个节点。若有节点地址相等，则表明该单链表有循环，反之则不存在循环。这种方法无疑效率比较低。而且如果出现下面这种情况的话，会更加麻烦。