双指针法关键在于数学原理理解

struct ListNode {
    int val;
    struct ListNode *next;
    ListNode(int x) :
        val(x), next(NULL) {
    }
};
*/
class Solution {
public:
    ListNode* EntryNodeOfLoop(ListNode* pHead) {
        ListNode *fast=pHead,*slow=pHead;
        while(fast){
            if(fast->next==nullptr){
                return NULL;
            }
            slow=slow->next;
            fast=fast->next->next;
            if(fast==slow){
                fast=pHead;
                while(fast!=slow){
                    fast=fast->next;
                    slow=slow->next;
                }
                return fast;
            }
        }
        return NULL;
    }