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) {
//         unordered_set<ListNode*> s;
//         ListNode* cur_pHead=pHead;
//         while(cur_pHead){
//             if(s.find(cur_pHead)==s.end()){
//                 s.insert(cur_pHead);
//                 cur_pHead=cur_pHead->next;
//             }else{
//                 return cur_pHead;
//             }
//         }
//                     return nullptr;
        
        ListNode* fast=pHead,*slow=pHead;
        while(fast && fast->next){
            fast=fast->next->next;
            slow=slow->next;
            if(fast==slow)
                break;
        }
        if(!fast || !fast->next)
            return nullptr;
        fast=pHead;
        while(fast!=slow){
            fast=fast->next;
            slow=slow->next;
        }
        return fast;
    }
};