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;
    }
}