题解 | #判断链表中是否有环#

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
//时间复杂度：O(n)，空间复杂度：O(n)需要把所有的节点都存放在map中
import java.util.*;
public class Solution {
    public boolean hasCycle(ListNode head) {
        if(head==null){
            return false;
        }
        Map<ListNode,Integer> map = new HashMap<>();
        while(head!=null){
            if(map.containsKey(head)){
                return true;
            }
            map.put(head,head.val);
            head = head.next;
        }
        return false;
    }
}


//使用快慢指针，时间复杂度：O(n)，空间复杂度：O(1)
    public boolean hasCycle(ListNode head) {
        if(head==null){
            return false;
        }
        ListNode slow = head;
        ListNode fast = head;
        while(slow!=null&&fast!=null){
            slow = slow.next;
            if(fast.next!=null){
                fast = fast.next.next;
            }else{
                return false;
            }
            if(slow==fast)
                return true;
        }
        return false;
    }