Leetcode: Linked List Cycle II

Given a linked list, return the node where the cycle begins. If there is no cycle, return null.

Follow up:
Can you solve it without using extra space?

一次过，还是Runner Technique的方法

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode detectCycle(ListNode head) {
        if (head == null || head.next == null) return null;
        ListNode runner = head;
        ListNode walker = head;
        while (runner != null && runner.next != null) {
            runner = runner.next.next;
            walker = walker.next;
            if (runner == walker) break;
        }
        if (runner != walker) return null;
        runner = head;
        while (runner != walker) {
            runner = runner.next;
            walker = walker.next;
        }
        return walker;
    }
}