判断单向链表是否有环

1.设立2个指针i，j指向头结点

2.i走1步，j走2步.如果有环，j一定能追上i；

3.如果j不为空，且i和j相等此链表即为有环。

 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 }