链表中环的入口结点

参考思考：https://blog.nowcoder.net/n/deaa284f105e48f49f38b5d7cb809cd7
代码:

/*
struct ListNode {
    int val;
    struct ListNode *next;
    ListNode(int x) :
        val(x), next(NULL) {
    }
};
*/
class Solution {
public:
    ListNode* EntryNodeOfLoop(ListNode* pHead)
    {
        if(pHead==NULL||pHead->next==NULL)
        {
            return NULL;
        }
        ListNode* fast = pHead;
        ListNode* slow = pHead;
        while(fast!=NULL&&fast->next!=NULL)
        {
            fast = fast->next->next;
            slow = slow->next;
            if(fast==slow)
            {
                ListNode* slow2 = pHead;
                while(slow2!=slow)
                {
                    slow2 = slow2->next;
                    slow = slow->next;
                }
                return slow2;
            }
        }
        return NULL;
    }
};