链表是否有环和寻找链表的入环点（快慢指针）

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

        if(pHead == nullptr || pHead->next == nullptr||pHead->next->next == nullptr){
            return nullptr;
        }

        ListNode* fast = pHead;
        ListNode* slow = pHead;

        bool isHave = false;

        // 判断是否有环
        while(fast!= nullptr && fast->next != nullptr){
            fast = fast->next->next;
            slow = slow->next;
            if(fast == slow){
                isHave = true;
                break;
            }
        }

        //有环之后，快指针回到头节点一次走一步；慢指针从原位置一次走一步；相遇点就是入环点
        if(isHave){
            fast = pHead;
            while(true){
                if(fast == slow){
                    break;
                }
                fast = fast->next; 
                slow = slow->next;
            }
            return slow;
        }
        else{
            return nullptr;
        }
    }
};