leetcode141. 环形链表

1：快慢链表(空间O1，时间On)

class Solution {
public:
    bool hasCycle(ListNode *head) {
        ListNode *fast = head;
        ListNode *slow = head;
        while(fast!=NULL&&fast->next!=NULL){
            fast = fast->next->next;
            slow = slow->next;
            if(slow==fast){
                return true;
            }
        }
        return false;
    }
};

2：时间O1，空间O1

bool hasCycle(ListNode *head) {
    int count = 10001;
    while(count){
        if(head==NULL)
            return false;
        head = head->next;
        count--;
    }
    return true;
}