Linked List Cycle II
Given the head of a linked list, determine whether the linked list contains a cycle.
If a cycle exists, return the node where the cycle begins.
If there is no cycle, return null.
A cycle exists when a node's next pointer points to a previous node in the linked list.
Example 1
Input
head = [3, 2, 0, -4] pos = 1
Output
2
Explanation
The last node points back to the node containing 2.
Therefore, the cycle begins at node 2.
Example 2
Input
head = [1, 2] pos = 0
Output
1
Explanation
The last node points back to the first node.
Therefore, the cycle begins at node 1.
Constraints
Hints:
Hint 1
Use two pointers:
- slow moves one step at a time.
- fast moves two steps at a time.
Hint 2
If slow and fast meet, a cycle exists.
If fast reaches null, there is no cycle.
Author & Technical Reviewer
Technically reviewed by: ExamAdda Technical Review Team
Technical Reviewers, ExamAdda
Software engineers at ExamAdda who check every article's definitions, complexity claims and code examples before and after publishing.
Published
Aug 26, 2026
Last updated
Aug 26, 2026
Content Verification Methodology
Definitions and complexity claims were checked against authoritative computer-science references. Code examples were compiled and tested with standard, boundary and edge-case inputs.
Expected Output