Longest Consecutive Sequence
Given an unsorted array of integers nums, return the length of the longest consecutive elements sequence.
You must write an algorithm that runs in O(n) time.
Example 1
Input
6 100 4 200 1 3 2
Output
4
Explanation
The longest consecutive sequence is 1, 2, 3, 4. Therefore, its length is 4.
Example 2
Input
10 0 3 7 2 5 8 4 6 0 1
Output
9
Explanation
The longest consecutive sequence is 0, 1, 2, 3, 4, 5, 6, 7, 8. Therefore, its length is 9.
Constraints
Hints:
Hint 1
Store all numbers in a hash set for O(1) lookups.
Hint 2
Start building a sequence only if the current number has no predecessor (num - 1). Then count consecutive numbers to find the longest sequence.
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 6, 2026
Last updated
Aug 7, 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