128-longest_consecutive_sequence.py

"""

https://leetcode.com/problems/longest-consecutive-sequence/

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

"""

def longestConsecutive(nums: list[int]) -> int:
    numsSet = set(nums)
    longest = 0
    for num in nums:
        if (num - 1) not in numsSet:
            length = 0
            while (num + length) in numsSet:
                length += 1
            longest = max(length, longest)
    return longest

if __name__ == '__main__':
    nums = [100,4,200,1,3,2]
    print(longestConsecutive(nums))