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128. Longest Consecutive Sequence

Array Hash Table Union Find

Problem - 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

Example 3:

Input: nums = [1,0,1,2]
Output: 3

 

Constraints:

  • 0 <= nums.length <= 105
  • -109 <= nums[i] <= 109

Solutions

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

        for num in hash_table:
            if num - 1 not in hash_table:
                val = num + 1
                while val in hash_table:
                    val += 1
                result = max(result, val - num)

        return result

Submission Stats:

  • Runtime: 43 ms (88.18%)
  • Memory: 34.3 MB (29.62%)