class Solution {
public:
ListNode *entryNodeOfLoop(ListNode *head) {
ListNode *slow = head;
ListNode *quick = head;
while(quick && quick->next){
slow = slow->next;
quick = quick->next->next;
if(quick == slow) break;
}
if(!quick || !quick->next) return NULL;
slow = head;
while(slow != quick){
slow = slow->next;
quick = quick->next;
}
}
};