141.java

/**
 * 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) {
        // // 采用集合的方法，复杂度太高
        // Set<ListNode> set = new HashSet<>();

        // ListNode curr = head;
        // while(curr!=null){
        //     if(set.contains(curr)){
        //         return true;
        //     }
        //     set.add(curr);
        //     curr=curr.next;
        // }
        // return false;

        // 双指针法

        ListNode fast = head;
        ListNode slow = head;


        while(fast!=null &&fast.next!=null){
            
            slow=slow.next;
            fast=fast.next.next;
            if(fast==slow){
                return true;
            }
        }
        return false;
        
    }
}

// 第二次做法
/**
 * 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 slow = head;
        ListNode fast = head;
        while(fast!=null&&fast.next!=null){
            slow = slow.next;
            fast = fast.next.next;
            if(slow == fast){
                return true;
            }
        }
        return false;
    }
}

// 第三次做法
/**
 * 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 slow = head;
        ListNode fast = slow;
        while(slow!=null&&fast!=null&&fast.next!=null){
            slow = slow.next;
            fast = fast.next.next;
            if(slow==fast){
              return true;  
            }
        }
        return false;

    }
}