鏈表是否有環

轉載自：http://blog.csdn.net/wangshihui512/article/details/9034157。
有環的鏈表指的是鏈表有環路，例如下面：A->B->C->D->B，這樣遍歷的時候B->C->D->B就形成一個環路一直循環。

#include <cstdio>  

typedef struct list  
{  
    int data;  
    struct list *next;  
}LIST;  

/* Method 1: check the occurrence of p->next from head to p */  
bool check_circle_1(LIST *head)  
{  
    LIST *p = head, *q= NULL;  

    if (p == NULL)  
        return false;  

    while (p->next)  
    {  
        /* check whether p points to itself */  
        if (p->next == p)  
        {  
            return true;  
        }  

        /* check the occurrence of p->next in head to p */  
        q = head;  
        while (q != p)  
        {  
            if (q == p->next)  
            {  
                return true;  
            }  
            q = q->next;  
        }  
        p = p->next;  
    }  
    /* p->next is NULL, not a circle */  
    return false;  
}  

/* Method 2: q goes faster than p, if at last p == q, means there's circle */  
/* 優點：邏輯上簡單。 缺點：無法具體知道從哪個點拆開該圈 */  
bool check_circle_2(LIST *head)  
{  
    LIST *p, *q;  
    p = head;  

    if (p == NULL)  
        return false;  

    q = p->next;  

    while (p != NULL && q != NULL)  
    {  
        if (p == q)  
        {  
            return true;  
        }  
        p = p->next;  
        if (q->next == NULL)  
        {  
            return 0;  
        }  
        else  
        {  
            q=q->next->next;  
        }  
    }  
    return 0;  
}  
void testIS()  
{  
    LIST a, b, c, *head = &a;  
    a.next = &b;  
    b.next = &c;  
    c.next = &a;  
    printf("1:is circle | 0:not circle || result check_circle_1(head):\  
        %d\n", check_circle_1(head));  
    printf("1:is circle | 0:not circle || result check_circle_1(head):\  
        %d\n", check_circle_2(head));  
}  
void testNO()  
{  
    LIST a, b, c, *head = &a;  
    a.next = &b;  
    b.next = &c;  
    c.next = NULL;  
    printf("1:is circle | 0:not circle || result check_circle_1(head):\  
        %d\n", check_circle_1(head));  
    printf("1:is circle | 0:not circle || result check_circle_1(head):\  
        %d\n", check_circle_2(head));  
}  
int main()  
{  
    printf("-------testIS()--------\n");  
    testIS();  
    printf("-------testNO()--------\n");  
    testNO();  
}  
/**************************************** 
程序運行結果如下： 
-------testIS()-------- 
1:is circle | 0:not circle || result check_circle_1(head):        1 
1:is circle | 0:not circle || result check_circle_1(head):        1 
-------testNO()-------- 
1:is circle | 0:not circle || result check_circle_1(head):        0 
1:is circle | 0:not circle || result check_circle_1(head):        0 

Process returned 0 (0x0)   execution time : 0.078 s 
Press any key to continue. 

*****************************************/