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128. Longest Consecutive Sequence

Medium

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

class Solution:
  def longestConsecutive(self, nums: List[int]) -> int:
    res = 0
    seen = set(nums)

    for num in nums:
      if num - 1 not in seen:
          length = 0

          while num in seen:
            num += 1
            length += 1

          res = max(res, length)

    return res