Given an unsorted array of integers, find the length of the longest consecutive elements sequence.

For example, given [100, 4, 200, 1, 3, 2], the longest consecutive element sequence is [1, 2, 3, 4]. Return its length: 4.

Algorithm should run in O(n) complexity.

JAVA

import java.util.HashSet;

public class LongestConsecutiveSequence {
    public int longestConsecutive(int[] nums) {
        HashSet<Integer> set = new HashSet<>();
        for (int num : nums) {
            set.add(num);
        }
        int longestStreak = 0;
        for (int num : set) {
            if (!set.contains(num - 1)) {
                int currentNum = num;
                int currentStreak = 1;
                while (set.contains(currentNum + 1)) {
                    currentNum++;
                    currentStreak++;
                }
                longestStreak = Math.max(longestStreak, currentStreak);
            }
        }
        return longestStreak;
    }

    public static void main(String[] args) {
        int[] nums = {100, 4, 200, 1, 3, 2};
        LongestConsecutiveSequence solution = new LongestConsecutiveSequence();
        System.out.println(solution.longestConsecutive(nums)); // Output: 4
    }
}

JAVASCRIPT

function longestConsecutive(nums) {
    const set = new Set(nums);
    let longestStreak = 0;
    for (const num of set) {
        if (!set.has(num - 1)) {
            let currentNum = num;
            let currentStreak = 1;
            while (set.has(currentNum + 1)) {
                currentNum++;
                currentStreak++;
            }
            longestStreak = Math.max(longestStreak, currentStreak);
        }
    }
    return longestStreak;
}

const nums = [100, 4, 200, 1, 3, 2];
console.log(longestConsecutive(nums)); // Output: 4

PYTHON

def longest_consecutive(nums):
    num_set = set(nums)
    longest_streak = 0
    for num in num_set:
        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

nums = [100, 4, 200, 1, 3, 2]
print(longest_consecutive(nums)) # Output: 4