Leetcode: 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?

Analysis: typical Runner Technique. 一次过

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