128. Longest Consecutive Sequence

• categories: Code, Interview Question, leetcode, Medium
• source: https://leetcode.com/problems/longest-consecutive-sequence
• topics: Hash Table

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

Idea

Use set for O(1) lookup

class Solution:
    def longestConsecutive(self, nums: List[int]) -> int:
        longest_streak = 0
        num_set = set(nums)
 
        for num in num_set:
            if num - 1 not in num_set:
                current_num = num
                current_streak = 1
 
                while current_num + 1 in num_set:
                    current_num += 1
                    current_streak += 1
 
                longest_streak = max(longest_streak, current_streak)
 
        return longest_streak