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?

判断一个链表是否有环

C++（10ms）：

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