Example 1
Input
longest_consecutive([100, 4, 200, 1, 3, 2])
Output
4
The longest consecutive sequence is [1, 2, 3, 4], so the length is 4.
Full lesson preview
Find the longest consecutive sequence
Given an unsorted list of integers, find the length of the longest run of consecutive integers. Use hashing for O(n) time.
Python practice15 minHashing & SetsIntermediateLast updated March 26, 2026
Problem statement
You are given an unsorted list of integers nums. Return the length of the longest sequence of consecutive integers that can be formed from elements of nums. The sequence elements must be consecutive integers, but the order in the input does not matter. Aim for O(n) average time complexity using a hash-based approach.
Task
Practice using sets for O(1) membership checks to compute the longest consecutive sequence in an array.
Examples
Input format
A list of integers: nums
Output format
An integer representing the length of the longest consecutive sequence
Constraints
0 <= len(nums) <= 10^5; integers may be negative; aim for O(n) average time using a set
Samples
Sample 1
Input
[100, 4, 200, 1, 3, 2]
Output
4
Longest run is [1,2,3,4] -> length 4