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?

双指针问题，一个runner 和一个walker，runner 每次向后移动两个，walker 每次向后移动一个。当walker 等于runner 时，说明存在闭环，如果runner 或者 runner 的next 为 null，说明不存在闭环。代码如下：

/**
* 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 fastNode = head;
ListNode slowNode = head;
while (fastNode != null && fastNode.next != null) {
fastNode = fastNode.next.next;
slowNode = slowNode.next;
if (slowNode == fastNode) {
return true;
}
}
return false;
}
}