LeetCode Linked List Cycle II

原题链接在这里：https://leetcode.com/problems/linked-list-cycle-ii/

首先找到是否有cycle，若是没有返回null。

若是有cycle，可以从head，用two pointers一个一个试直到找到cycle beginner，Time O(n^2).

另外一种方法参见了这篇帖子，是数学想法。

AC Java:

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