LEETCODE--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?

判断链表是否有环：使用追赶的方法，设定两个指针slow、fast，从头指针开始，每次分别前进1步、2步。如存在环，则两者相遇；如不存在环，fast遇到NULL退出。

/**

* 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 *slow = head;

ListNode *fast = head;

while(head){

if(fast->next && fast->next->next){

fast = fast->next->next;

}else{

return false;

}

slow = slow->next;

if(slow == fast)

return true;

}

return false;

}

};