---
title: '128. Longest Consecutive Sequence'
description: Given an unsorted array of integers nums, return the length of the longest consecutive elements sequence
icon: dot
topics:
  - { name: "Array", slug: "array" }
  - { name: "Hash Table", slug: "hash-table" }
  - { name: "Union-Find", slug: "union-find" }
issue: "https://github.com/prdlk/leetcode/issues/34"
sidebar:
  label: 'Longest Consecutive Sequence'
  badge: 'Medium'
---

::::warning
You must write an algorithm that runs in O(n) time.
::::

### Example 1:
- Input: `nums = [100,4,200,1,3,2]`
- Output: `4`
- Explanation: The longest consecutive elements sequence is [1, `2`, `3`, 4]. Therefore its length is `4`.

### Example 2:
- Input: `nums = [0,3,7,2,5,8,4,6,0,1]`
- Output: `9`

### Example 3:
- Input: `nums = [1,0,1,2]`
- Output: `3`

### Constraints:

- `0 <= nums.length <= 10^5`
- `-10^9 <= nums[i] <= 10^9`

## Solution

```py
class Solution:
    def longestConsecutive(self, nums: list[int]) -> int:
        seen = set(nums)
        best = 0

        for n in seen:
            if n - 1 in seen:
                continue

            length = 1
            while length + n in seen:
                length += 1

            best = max(length, best)

        return best
```