141 Linked List Cycle

---

title: Linked List Cycle
tags:
- linked-list-cycle
- No.141
- simple
- list
- pointer

---

Description

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

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

Corner Cases

• empty list

Solutions

Fast and Slow Pointers

Set a fast pointer and a slow one. When the slow meets the fast, then a cycle exists.

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