141. Linked List Cycle

my code that destroy the list

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    bool hasCycle(ListNode *head) {
        ListNode *curr = head;
        if (head == NULL)return false;
        while(curr->next) {
            ListNode *tmp = curr->next;
            if (tmp == head)return true;
            curr->next = head;
            curr = tmp;
        }
        return false;
    }
};

tortoise and hare algorithm

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    bool hasCycle(ListNode *head) {
        if (head == NULL || head->next == NULL)return false;
                            //pay attention not forget to see ifnull head->next

        ListNode *tortoise = head;
        ListNode *hare = head;

        while(hare->next && hare->next->next) {
            //should first ensure that hare->next is not NULL then hare->next has next
            tortoise = tortoise->next;
            hare = hare->next->next;
            if (tortoise == hare)return true;
        }
        return false;
    }
};