LeetCode 141. Linked List Cycle

Given a linked list, determine if it has a cycle in it.

Follow up:
Can you solve it without using extra space?

update v2.0:
//后来想到了一个不赖皮的方法 用 set 存储 判断每次存入 next 结点后 set 的 size()有没有发生变化
//没有发生变化说明这个结点已经被存储过了 就说明是个环
/**
 * 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)
            return false;
        set<ListNode *> s;
        ListNode *t;
        t = head;
        int cnt = 0;
        while(t->next != NULL) {
            s.insert(t);
            if(cnt == s.size()) {
                return true;
            }
            cnt = s.size();
            t = t->next;
        }
        return false;
    }
};



version 1.0:
//感觉自己做这道题的方法有点赖皮。。。唔。。。
/**
 * 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)
            return false;
        ListNode *p;
        p = head;
        p->val = 99999999;
        while(p->next != NULL) {
            p = p->next;
            if(p->val != 99999999) {
                p->val = 99999999;
            } else {
                return true;
            }
        }
        return false;
    }
};

update v2.0:

//后来想到了一个不赖皮的方法用 set 存储判断每次存入 next 结点后 set 的 size()有没有发生变化

//没有发生变化说明这个结点已经被存储过了就说明是个环

/**

* 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)

return false;

set<ListNode *> s;

ListNode *t;

t = head;

int cnt = 0;

while(t->next != NULL) {

s.insert(t);

if(cnt == s.size()) {

return true;

}

cnt = s.size();

t = t->next;

}

return false;

}

};

version 1.0:

//感觉自己做这道题的方法有点赖皮。。。唔。。。

/**

* 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)

return false;

ListNode *p;

p = head;

p->val = 99999999;

while(p->next != NULL) {

p = p->next;

if(p->val != 99999999) {

p->val = 99999999;

} else {

return true;

}

return false;

}

};