java单链表快慢指针

例题：
LeetCode876. 链表的中间结点

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

快慢指针真香！不急还有一个例题
LeetCode141. 环形链表
这个是官方题解

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

超级简单~~