leetcode：142. Linked List Cycle II（Java）解答

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原文地址：http://blog.csdn.net/u012975705/article/details/50412899
题目地址：https://leetcode.com/problems/linked-list-cycle-ii/

Linked List Cycle II

Given a linked list, return the node where the cycle begins. If there is no cycle, return null.Note: Do not modify the linked list.Follow up:Can you solve it without using extra space?

解法（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 slow = head;        ListNode fast = head.next;        while (slow != null && fast != null && slow != fast) {            // if (slow == fast) {            //     temp = slow;            //     break;            // }            slow = slow.next;            fast = fast.next == null ? fast.next : fast.next.next;        }        if (slow == fast) {            slow = head;            fast = fast.next;            while (slow != fast) {                slow = slow.next;                fast = fast.next;            }            return slow;        }        return null;    }}