题解 | #链表中环的入口结点#

/*
struct ListNode {
    int val;
    struct ListNode *next;
    ListNode(int x) :
        val(x), next(NULL) {
    }
};
*/
class Solution {
public:
    ListNode* hasCycle(ListNode *head) 
    {
        ListNode* fast = head;
        ListNode* slow = head;
        while(fast && fast->next)
        {
            fast = fast->next->next;
            slow = slow->next;

            if(fast == slow)
            {
                return slow;
            }
        }
        return nullptr;
    }
    ListNode* EntryNodeOfLoop(ListNode* pHead) 
    {
        ListNode* slow = hasCycle(pHead);
        if(slow == nullptr)
        {
            return NULL;
        }

        ListNode* head = pHead;
        while(head != slow)
        {
            head = head->next;
            slow = slow->next;
        }
        return head;

    }
};