Linked_List_Cycle.cpp

/**
 ** 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 = head;
		ListNode *q = head;
		
		while(true) {
			if(p->next == NULL)
				return false;
			p = p->next;

			if(q->next == NULL) {
				return false;
			} else if(q->next->next == NULL) {
				return false;
			}
			q = q->next->next;

			if(p == q)
				return true;
		}
    }
};