leetcode141. Linked List Cycle

141. Linked List Cycle

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

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

解法

快慢指针法，当两指针重合，则有环。

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