Merge pull request #603 from harshitpandey-26/patch-4

Create LongestConsecutiveSequence.java

Author: Pradeepsingh61

Java/algorithms/arrays/LongestConsecutiveSequence.java

@@ -0,0 +1,39 @@
+// Author: HarshitKumarPandey
+// Date: 2025-10-09
+// Description: Find the length of the longest consecutive sequence using HashSet in O(n) time.
+
+import java.util.HashSet;
+
+public class LongestConsecutiveSequence {
+
+    public static int longestConsecutive(int[] nums) {
+        if (nums.length == 0) return 0;
+
+        HashSet<Integer> set = new HashSet<>();
+        for (int num : nums) set.add(num);
+
+        int longestStreak = 0;
+
+        for (int num : set) {
+            // Only start counting when it's the beginning of a sequence
+            if (!set.contains(num - 1)) {
+                int currentNum = num;
+                int currentStreak = 1;
+
+                // Count consecutive numbers
+                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};
+        System.out.println("Longest consecutive sequence length: " + longestConsecutive(nums));
+    }
+}