LeetCode Note Java 00141：Linked List Cycle

回傳輸入的 Linked List 是否形成 Cycle。

題目

Linked List Cycle Easy

Given head, the head of a linked list, determine if the linked list has a cycle in it.

There is a cycle in a linked list if there is some node in the list that can be reached again by continuously following the next pointer. Internally, pos is used to denote the index of the node that tail’s next pointer is connected to. Note that pos is not passed as a parameter.

Return true if there is a cycle in the linked list. Otherwise, return false.

我的解法

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

因為先寫過 00142 了，所以知道要確認有沒有形成 Cycle 是使用龜兔賽跑的方式判斷。