双指针简单 leetcode141. 环形链表

双指针

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

哈希表

public class Solution {
    public boolean hasCycle(ListNode head) {
        Set<ListNode> set = new HashSet<>();
        ListNode cur = head;
        while(cur != null){
            if(set.contains(cur)){
                return true;
            }
            set.add(cur);
            cur = cur.next;
        }
        return false;
    }
}