Leetcode:Linked List Cycle

经典题目：

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

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

方案一：

/**
 * 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* Node = NULL;
        ListNode* temp = NULL;
        Node = head;
        while(head->next!=NULL)
        {
            temp = head;
            head = head->next;
            temp->next = Node;
            if(head->next==Node)
                return true;
        }
        return false;
    }
};

方案二：

/**
 * 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) {
        // head ==null?
        if(head==NULL) return false;
        ListNode *fast = head->next, *slow=head;
        while(fast!=NULL && fast->next!=NULL)
        {
            if(fast==slow)
                return true;
            fast = fast->next->next;
            slow = slow->next;
        }
        return false;
    }
};

建议与易出错地方：

1.在纸上写代码时，开始留些空白，如果需要处理边界情况，可直接加上。

2. 声明指针变量时一定要保持*在变量上的好习惯，否则容易写成 ListNode* fast=head, slow = head; 这就错了！

3. 过程中一个错是  开始初始化直接

 ListNode *fast = head, *slow=head;
 后面fast==slow的判断，就直接为真了。