Longest Consecutive Sequence in Java: Explanation & Practice

Find length of longest consecutive elements sequence

Problem summary

Given an unsorted array, find the length of the longest consecutive elements sequence. Use HashSet for O(n) solution.

Starter code

public class Main {
    public static void main(String[] args) {
        int[] nums = {100, 4, 200, 1, 3, 2};
        
        // Add all to HashSet
        // Only start counting from sequence start (n-1 not in set)
        // Count consecutive numbers
        // Print: Length: 4
    }
}

Expected output and test cases

[100,4,200,1,3,2] → 1,2,3,4
```
Length: 4
```
[0,3,7,2,5,8,4,6,0,1] → 0-8
```
Length: 9
```
[] → 0
```
Length: 0
```

Hints

Put all numbers in HashSet for O(1) lookup
Only start counting from sequence beginning (num-1 not in set)
This ensures we count each sequence only once
Count consecutive numbers using set.contains(num+1)

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