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* EntryNodeOfLoop(ListNode* pHead) {
        ListNode* fast=pHead;
        ListNode* slow=pHead;
        while(fast!=nullptr){
            slow=slow->next;
            if(fast->next==nullptr){
                return nullptr;
            }
            fast=fast->next->next;
            if(slow==fast){
                ListNode* ptr=pHead;
                while(ptr!=slow){
                    ptr=ptr->next;
                    slow=slow->next;
                }
                return ptr;
            }
        }

       return nullptr;
    }
};