128-longest-consecutive-sequence

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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(Python)

class Solution:
    def longestConsecutive(self, nums):

        longest_streak = 0
        num_set = set(nums)

        for num in nums:
            if num-1 not in num_set:
                current_streak = 1
                cur_num = num
                while cur_num+1 in num_set:
                    cur_num += 1
                    current_streak += 1
                longest_streak = max(longest_streak,current_streak)

        return longest_streak