see-141. Linked List Cycle

查找链表中是否有环：分别定义一个快指针、慢指针；

移动指针，快指针每次走二步，慢指针每次走一步，途中若快指针先抵达尾部，那么no cycle；若快慢指针相遇，那么有环。

/**
 * 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 0;  //no cycle
        
        ListNode *fast=head;
        ListNode *low=head;
        
        while(fast!=NULL&&low!=NULL)
        {

            low=low->next; // low go 1 step

            for(int i=1;i<3;i++)  //fast go 2 steps
            {
                if(fast==NULL)
                    return 0;
                else
                    fast=fast->next;
            }
            
            if(fast==low)
                return 1;
            
        }
        
        return 0; //fast meet end of the link list, there is no cycle
    }
};