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128. Longest Consecutive Sequence

Difficulty Topics

Description

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

Solution

longest-consecutive-sequence.py
class Solution:
    def longestConsecutive(self, nums: List[int]) -> int:
        s = set(nums)
        res = 0

        while s:
            first = last = s.pop()

            while first - 1 in s:
                s.remove(first - 1)
                first -= 1

            while last + 1 in s:
                s.remove(last + 1)
                last += 1

            res = max(res, last - first + 1)

        return res