链表是否有环

转载自：http://blog.csdn.net/wangshihui512/article/details/9034157。
有环的链表指的是链表有环路，例如下面：A->B->C->D->B，这样遍历的时候B->C->D->B就形成一个环路一直循环。

#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 */  /* 优点：逻辑上简单。 缺点：无法具体知道从哪个点拆开该圈 */  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();  }  /**************************************** 程序运行结果如下： -------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. *****************************************/