Linked List Cycle

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

Follow up:
Can you solve it without using extra space?

描述

判断一个链表是否是环，要求不能使用额外空间辅助

分析

设计一个快指针和一个慢指针，快指针每次走两步，慢指针每次走一步，若两个指针相遇，且指针不为空，则存在环

代码

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/**
 * 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) {

        ListNode fast, slow;
        fast = slow = head;

        while (fast != null && fast.next != null) {

            fast = fast.next.next;
            slow = slow.next;

            if (fast == slow) {
                return true;
            }
        }

        return false;
    }
}