# LeetCode 128: Longest Consecutive Sequence — Step-by-Step Visual Trace

**Medium** — Array | Hash Table | Union Find

## The Problem

Given an unsorted array of integers, find the length of the longest sequence of consecutive elements. The algorithm must run in O(n) time complexity.

## Approach

Convert the array to a set for O(1) lookups, then for each number that starts a sequence (has no predecessor), count consecutive numbers forward. This ensures each number is visited at most twice, maintaining O(n) time complexity.

**Time:** O(n) · **Space:** O(n)

## Code

```python
class Solution:
    def longestConsecutive(self, nums: List[int]) -> int:
        num_set = set(nums)
        max_length = 0

        for num in num_set:
            if num - 1 not in num_set:
                current_num = num
                current_length = 1

                while current_num + 1 in num_set:
                    current_num += 1
                    current_length += 1

                max_length = max(max_length, current_length)

        return max_length
```

## Watch It Run

> **[Open interactive visualization](https://tracelit.dev/app?trace=0128_longest-consecutive-sequence)**

> **Try it yourself:** Open [TraceLit](https://tracelit.dev/app?trace=0128_longest-consecutive-sequence) and step through every line.

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