141. Linked List Cycle(判断链表是否有环)

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?

 利用快慢指针，如果相遇则证明有环

注意边界条件： 如果只有一个node.

public class Solution {
    public boolean hasCycle(ListNode head) {
        if(head==null || head.next==null) return false;
        ListNode slower =head,faster = head;
        while(faster!=null && faster.next!=null){
            if(faster==slower) return true;
            faster = faster.next.next;
            slower = slower.next;
        }
        return false;
    }
}