[Solved] 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.

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Question

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

Python Solution

from heapq import heappush,heappop,heapify

class Solution:
    def longestConsecutive(self, nums: List[int]) -> int:
        nums = list(set(nums))
        heapify(nums)
        prev = -float('inf')
        r = 1
        m = 0
        i = len(nums)-1
        
        while i!=-1:
            curr = heappop(nums)
            if prev+1==curr:
                r+=1
            else:
                r = 1
            
            m = max(m,r)
            prev = curr
            i-=1
        
        return m

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