LongestConsecutiveSequence.py

"""
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: 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


Constraints:

0 <= nums.length <= 105
-109 <= nums[i] <= 109
"""
from typing import List


class Solution:
    def longestConsecutive(self, nums: List[int]) -> int:
        if not nums:
            return 0

        num_set = set(nums)
        longest_streak = 0

        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


# Test case
solution = Solution()
nums = [100, 4, 200, 1, 3, 2]
result = solution.longestConsecutive(nums)
print(result)  # Expected output: 4