Introduction

In this blog post, we will explore how to solve the LeetCode problem "128" using AI. LeetCode is a popular platform for preparing for coding interviews, and with the help of AI tools like Chatmagic, we can generate solutions quickly and efficiently - helping you pass the interviews and get the job offer without having to study for months.

Problem Statement

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

Explanation

Here's a solution to find the length of the longest consecutive elements sequence in an unsorted array of integers, meeting the O(n) time complexity requirement.

• Use a Set: Convert the input array into a set. This allows for O(1) average-case time complexity for checking the existence of an element.
• Iterate and Extend: Iterate through the original array. For each number, check if it's the start of a sequence (i.e., the number minus 1 is not in the set). If it is, extend the sequence as far as possible by incrementing the current number and checking for its existence in the set.

• Track Maximum Length: Keep track of the maximum sequence length found so far.

• Runtime Complexity: O(n), Storage Complexity: O(n)

Code

    def longestConsecutive(nums):
    """
    Finds the length of the longest consecutive elements sequence in an unsorted array.

    Args:
        nums: A list of integers.

    Returns:
        The length of the longest consecutive elements sequence.
    """

    num_set = set(nums)
    longest_streak = 0

    for num in nums:
        # Check if 'num' is the start of a sequence
        if (num - 1) not in num_set:
            current_num = num
            current_streak = 1

            while (current_num + 1) in num_set:
                current_num += 1
                current_streak += 1

            longest_streak = max(longest_streak, current_streak)

    return longest_streak