141. 环形链表

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */
bool hasCycle(struct ListNode *head) {
    struct ListNode *fast_p=head,*slow_p=head;
    while(fast_p!=NULL&&fast_p->next!=NULL){
        slow_p=slow_p->next;
        fast_p=fast_p->next->next;
        if(slow_p==fast_p) {
            return true;   
        }
    }
    return false;
}