數據結構——判斷單鏈表是否有環

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

#define INSERT_NUM 100
#define FAST_POINT_STEP 2

typedef int ElemType;

typedef struct LNode{
    ElemType data;
    struct LNode *next;
}LinkList, *pNode;

//init the linklist
void initList(pNode *head){
    (*head) = (LinkList*)malloc(sizeof(LinkList));
    (*head)->next = NULL;
    (*head)->data = 9;
}

//insert Element
int 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 one
void 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 loop
void 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");

}

        之前在網上看到有人說，判斷一個鏈表是否有環，還可以使用兩個for循環嵌套的方法，當外層循環步進一個節點時，內層循環就遍歷外層循環節點之後的所有節點，然後比較內外循環的兩個節點。若有節點地址相等，則表明該單鏈表有循環，反之則不存在循環。這種方法無疑效率比較低。而且如果出現下面這種情況的話，會更加麻煩。