如何判断单链表是否成环以及查找环的接入点

首先定义一个判断工具

typedef unsigned char Boolean;#define TURE 1#define FLASE 0

1、是否带环判断：使用追赶的方法，设定两个指针slow、first，从头指针开始，每次分别前进1步、2步。如存在环，则两者相遇；如不存在环，fast遇到NULL退出。

Boolean FindListTail(PList &head, PList &hNode) {Boolean OK = FLASE;    Node *slow, *fast;    slow = fast = head;    while(fast != NULL && fast -> next != NULL) {        fast = fast -> next -> next;        slow = slow-> next;        if(fast == slow) {            hNode = fast;            OK = TURE;        }    }    return OK;}

环的接入点：碰撞点p到连接点的距离=头指针到连接点的距离，因此，分别从碰撞点、头指针开始走，相遇的那个点就是连接点。

Node *findLoopPort(PList head) {if(head == NULL || head-> next == NULL)         return NULL;    Node *slow,*fast;    slow = fast = head;    while(fast != NULL && fast-> next != NULL) {        fast = fast-> next-> next;        slow = slow -> next;        if(fast == slow)            break;    }    if(fast != slow)         return NULL;    fast = head;                   while(fast != slow) {        fast = fast -> next;        slow = slow -> next;    }    return fast;}