141. Linked List Cycle
Easy
Given a linked list, determine if it has a cycle in it.
To represent a cycle in the given linked list, we use an integer pos
which represents the position (0-indexed) in the linked list where tail connects to. If pos
is -1
, then there is no cycle in the linked list.
Example 1:
Input: head = [3,2,0,-4], pos = 1
Output: true
Explanation: There is a cycle in the linked list, where tail connects to the second node.
Example 2:
Input: head = [1,2], pos = 0
Output: true
Explanation: There is a cycle in the linked list, where tail connects to the first node.
Example 3:
Input: head = [1], pos = -1
Output: false
Explanation: There is no cycle in the linked list.
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Follow up:
Can you solve it using O(1) (i.e. constant) memory?
package leetcode.easy; import java.util.HashSet; import java.util.Set; public class LinkedListCycle { @org.junit.Test public void test1() { ListNode ln1 = new ListNode(3); ListNode ln2 = new ListNode(2); ListNode ln3 = new ListNode(0); ListNode ln4 = new ListNode(-4); ln1.next = ln2; ln2.next = ln3; ln3.next = ln4; ln4.next = ln2; System.out.println(hasCycle1(ln1)); System.out.println(hasCycle2(ln1)); } @org.junit.Test public void test2() { ListNode ln1 = new ListNode(1); ListNode ln2 = new ListNode(2); ln1.next = ln2; ln2.next = ln1; System.out.println(hasCycle1(ln1)); System.out.println(hasCycle2(ln1)); } @org.junit.Test public void test3() { ListNode ln1 = new ListNode(1); ln1.next = null; System.out.println(hasCycle1(ln1)); System.out.println(hasCycle2(ln1)); } public boolean hasCycle1(ListNode head) { Set<ListNode> nodesSeen = new HashSet<>(); while (head != null) { if (nodesSeen.contains(head)) { return true; } else { nodesSeen.add(head); } head = head.next; } return false; } public boolean hasCycle2(ListNode head) { if (head == null || head.next == null) { return false; } ListNode slow = head; ListNode fast = head.next; while (slow != fast) { if (fast == null || fast.next == null) { return false; } slow = slow.next; fast = fast.next.next; } return true; } }