/* package whatever; // don't place package name! */
 
import java.util.*;
import java.lang.*;
import java.io.*;
import java.util.HashSet;
/* Name of the class has to be "Main" only if the class is public. */
class Ideone
{
 public static int longestConsecutive(int[] nums) {
        HashSet<Integer> set = new HashSet<>();
        for (int num : nums) set.add(num);
 
        int maxLength = 0;
 
        for (int num : set) {
            // Only start counting if it's the beginning of a sequence
            if (!set.contains(num - 1)) {
                int currentNum = num;
                int currentStreak = 1;
 
                while (set.contains(currentNum + 1)) {
                    currentNum++;
                    currentStreak++;
                }
 
                maxLength = Math.max(maxLength, currentStreak);
            }
        }
 
        return maxLength;
    }
 
    public static void main(String[] args) {
        int[] nums = {100, 4, 200, 1, 3, 2};
        System.out.println("Longest consecutive sequence length: " + longestConsecutive(nums)); // Output: 4
    }
}
 
 
 
 

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