剑指offer:链表中环的入口结点

题目描述

一个链表中包含环，请找出该链表的环的入口结点。

/*
struct ListNode {
    int val;
    struct ListNode *next;
    ListNode(int x) :
        val(x), next(NULL) {
    }
};
*/
class Solution {
public:
    ListNode* EntryNodeOfLoop(ListNode* pHead)
    {
        if (pHead == NULL){
            return NULL;
        }

        int LoopLength = lengthOfLoop(pHead);

        if (LoopLength == 0){
            return NULL;
        }

        ListNode* faster = pHead;

        while (LoopLength--){
            faster = faster->next;
        }

        ListNode* slower = pHead;

        while (faster != slower){
            faster = faster->next;
            slower = slower->next;
        }

        return faster;
    }

private:
    int lengthOfLoop(ListNode* pHead){

        if (pHead == NULL){
            return 0;
        }

        ListNode* meetNode = NULL;

        ListNode* faster = pHead;
        ListNode* slower = pHead;

        while (faster){
            if (faster->next == NULL || faster->next->next == NULL){
                return 0;
            }

            faster = faster->next->next;
            slower = slower->next;

            if (faster == slower){
                meetNode = faster;
                break;
            }
        }

        int length = 1;
        slower = slower->next;

        while (slower != meetNode){
            slower = slower->next;
            length++;
        }

        return length;
    }

};