142. 环形链表 II

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *detectCycle(ListNode *head) {
        ListNode *f=head;
        ListNode *s=head;
        while(f!=NULL&&f->next!=NULL){
            s=s->next;
            f=f->next->next;
            if(s==f){
                ListNode *l1=f;
                ListNode *l2=head;
                while(l1!=l2){
                    l1=l1->next;
                    l2=l2->next;
                }
                return l1;
            }
        }
        return NULL;
    }
};