题解 | #判断链表中是否有环#

哈希方式

class Solution {
public:
    bool hasCycle(ListNode *head) {
        unordered_map<ListNode *, bool> m;
        while(head){
            if(m.find(head) != m.end())
                return true;
            m[head] = true;
            head = head->next;
        }
        return false;
    }
};

快慢指针

class Solution {
public:
    bool hasCycle(ListNode *head) {
        //快慢指针
        if(!head) return false;
        ListNode * slow = head;
        ListNode * fast = head->next;

        while(slow != fast && slow && fast){
            slow = slow->next;
            fast = fast->next;
            if(!fast) return false;
            fast = fast->next;
        }
        if(slow == fast)
            return true;
        return false;
    }
};