Leetcode 142环形链表II

class Solution {
public:
    ListNode* fisrtM(ListNode* head)
    {
        ListNode* slow;
        ListNode* faster;
        slow = faster = head;
        while(faster != NULL)
        {
            slow = slow->next;
            if(faster->next == NULL) return NULL;
            faster = faster->next->next;
            if(slow == faster) return slow;
        }
        return NULL;

    }
    ListNode *detectCycle(ListNode *head) {
        if(head == NULL) return NULL;
        ListNode* fm = fisrtM(head);
        if(fm == NULL) return NULL;
        ListNode* ans = head;
        while(ans != fm)
        {
            ans = ans->next;
            fm = fm->next;
        }
        return ans;
        
    }
};