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# 0142. Linked List Cycle II

<https://leetcode.com/problems/linked-list-cycle-ii>

## Description

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

There is a cycle in a linked list if there is some node in the list that can be reached again by continuously following the `next` pointer. Internally, `pos` is used to denote the index of the node that tail's `next` pointer is connected to (**0-indexed**). It is `-1` if there is no cycle. **Note that** `pos` **is not passed as a parameter**.

**Do not modify** the linked list.

**Example 1:**

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

```
**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.
```

**Example 2:**

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

```
**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.
```

**Example 3:**

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

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

**Constraints:**

* The number of the nodes in the list is in the range `[0, 104]`.
* `-105 <= Node.val <= 105`
* `pos` is `-1` or a **valid index** in the linked-list.

**Follow up:** Can you solve it using `O(1)` (i.e. constant) memory?

## ac

```java
/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode detectCycle(ListNode head) {
        // edge cases
        if (head == null || head.next == null) return null;

        // 2 pointers find meet point
        ListNode slow = head, fast = head, meetNode = null;
        while (fast != null && fast.next != null) {
            fast = fast.next.next;
            slow = slow.next;
            if (fast == slow) {
                meetNode = slow;
                break;
            }
        }
        if (meetNode == null) return null; // no cycle

        slow = head;
        while (slow != meetNode) {
            slow = slow.next;
            meetNode = meetNode.next;
        }

        return meetNode;
    }
}
```


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