判断单向链表是否有环

 1 #include <stdio.h>  
 2 #include <stdlib.h>  
 3 #include <string.h>  
 4 typedef struct student                                              //定义链表结构  
 5 {  
 6     int num;  
 7     struct student *pnext;  
 8 }stu,*pstu;  
 9 void link_tail_insert(pstu *phead,pstu *ptail,int i);                         
10 void link_show(pstu );      
11 void link_judge_loop(pstu phead);                                                    
12 void main(){  
13     pstu phead,ptail;  
14     int i;  
15     phead = NULL;  
16     ptail = NULL;  
17     while(scanf("%d",&i) != EOF){                                       
18         link_tail_insert(&phead,&ptail,i);  
19     }  
20     link_show(phead);   
21     ptail->pnext = phead;                                          //单链表建立环  
22     link_judge_loop(phead);                                            
23     system("pause");  
24   
25 }  
26 void link_tail_insert(pstu *phead,pstu *ptail,int i){             //尾插法建立链表  
27     pstu pnew;  
28     pnew = (pstu)malloc(sizeof(stu));  
29     memset(pnew,0,sizeof(stu));  
30     pnew->num = i;  
31     if(*ptail == NULL){  
32         *phead = pnew;  
33         *ptail = pnew;  
34     }  
35     else{  
36         (*ptail)->pnext = pnew;  
37         *ptail = pnew;  
38     }  
39 }  
40 void link_show(pstu phead){                                       //输出链表  
41     pstu pshow;  
42     pshow = phead;  
43     if(phead == NULL)  
44     {  
45         printf("no exsit\n");  
46         return;  
47     }  
48     while(pshow != NULL){  
49         printf("%d ",pshow->num);  
50         pshow = pshow->pnext;  
51     }  
52     putchar(‘\n‘);  
53 }  
54 void link_judge_loop(pstu phead){                               //判断是否有环  
55     pstu i,j;  
56     i = j = phead;  
57     while(j != NULL){   
58         i = i->pnext;  
59         j = j->pnext;  
60         if(j == NULL)                                           //j要分步走，判断是否为空，是空则结束循环  
61             break;  
62         j = j->pnext;                                          
63         if(i == j)                                               //i和j相等，有环  
64             break;  
65     }  
66     if(j != NULL && i == j)  
67         printf("link has loop\n");  
68     else  
69         printf("link has no  loop\n");  
70 }