【C语言】找带环单链表的环入口

思想：（1）快慢指针，先让快指针追上慢指针

            （2）令快指针指向表头，慢指针的指向不变，两个指针同时开始走

            （3）两个指针相遇时，所指节点即为环的入口

Node*FindEntry(Plinklist*pplist)

{

          assert(pplist);

          Node*fast = *pplist;

          Node*slow = *pplist;

          while(fast&&fast->next )  //让快指针追上慢指针

          {

                   fast = fast->next->next;

                   slow = slow->next;

                   if(fast == slow)

                   {

                             break;

                   }

          }

          fast = *pplist;

          while(fast != slow)

          {

                   fast = fast->next;

                   slow = slow->next;

          }

          returnfast;

}