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 }