> For the complete documentation index, see [llms.txt](https://mayanktyagi3111.gitbook.io/interview-prep/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://mayanktyagi3111.gitbook.io/interview-prep/linked-list/linked-list-cycle-ii.md).

# Linked List Cycle II

Given a linked list, return the node where the cycle begins. If there is no cycle, return `null`.

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.

**Note:** Do not modify the linked list.

**Example 1:**

```
Input: head = [3,2,0,-4], pos = 1
Output: tail connects to node index 1
Explanation: There is a cycle in the linked list, where tail connects to the second node.
```

![](https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist.png)

**Example 2:**

```
Input: head = [1,2], pos = 0
Output: tail connects to node index 0
Explanation: There is a cycle in the linked list, where tail connects to the first node.
```

![](https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist_test2.png)

**Example 3:**

```
Input: head = [1], pos = -1
Output: no cycle
Explanation: There is no cycle in the linked list.
```

![](https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist_test3.png)

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

```java
public class Solution {
    public ListNode checkLoop(ListNode head) {
        ListNode slow = head, fast = head;
        while (slow != null && fast != null && fast.next != null) {
            slow = slow.next;
            fast = fast.next.next;
            if (slow == fast)
                return fast;
        }
        return null;
    }

    public ListNode detectCycle(ListNode head) {
        if (head == null || head.next == null)
            return null;
        ListNode cycle = checkLoop(head);
        if (cycle == null)
            return cycle;
        ListNode temp = head;
        while (cycle != temp) {
            cycle = cycle.next;
            temp = temp.next;
        }
        return cycle;
    }
}
```