Linked List Cycle

Linked List Cycle

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

c++

/**
 * 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) {
         /*
        1. 空链表不成环
        2. 一个节点自环
        3. 一条链表完整成环
        */
        if(head==NULL)return false;
        if(head->next==NULL)return false;
        if(head->next==head)return true;

        ListNode* y=head->next;
        ListNode* x=head->next->next;
        while(x!=NULL && y!=NULL)
        {
            x=x->next;
            if(x==NULL)break;
            x=x->next;
            y=y->next;
            if(x==y)break;
        }
        return x==y;
    }

};

　　java版：

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *
 * **/

 public class Solution {
	public boolean hasCycle(ListNode head) {
		ListNode slow=head;
		ListNode fast=head;
		while(fast!=null&&fast.next!=null){
			slow=slow.next;
			fast=fast.next.next;
			if(slow==fast)
				return true;
		}
		return false;
	}

}