题解 | #判断链表中是否有环# 双针竞逐

import java.util.*;
/**
 * 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) return false;
        ListNode fast = new ListNode(-1), low = new ListNode(-1);
        fast.next = head;
        low.next = head;
        while(fast != null && low != null){
            if(low.next != null){
                low = low.next;
            }
            if(fast.next != null){
                fast = fast.next.next;
            }else{
                break;
            }
            if(fast == low) return true;
        }
        return false;
    }
}

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