added most optimal approach using HashSet, O(N) time, O(N) space solution

Author: NirmalSilwal

Leetcode/Youtube/LongestConsecutiveSequence.java

@@ -2,13 +2,18 @@
 
 import java.util.Arrays;
 import java.util.HashMap;
+import java.util.HashSet;
 import java.util.Map;
+import java.util.Set;
 
 public class LongestConsecutiveSequence {
 
 	// O(n^2) time, O(n) space
 	public static int lengthOfLongestConsecutiveSequence(int[] arr, int N) {
 
+		if (N == 0)
+			return 0;
+
 		Map<Integer, Integer> map = new HashMap<>();
 		for (int n : arr) {
 			if (!map.containsKey(n)) {
@@ -37,16 +42,24 @@ public static int lengthOfLongestConsecutiveSequence(int[] arr, int N) {
 	// approach 2 using Sorting
 	// O(NlogN) time, O(N) space
 	public static int lengthOfLongestConsecutiveSequence2(int[] arr, int N) {
+		if (N == 0)
+			return 0;
+
 		int max = 1;
 		int[] arrclone = arr.clone(); // making copy of original array
 
 		Arrays.sort(arrclone); // sorting array
 
 		int ans = 1;
 		for (int i = 1; i < arrclone.length; i++) {
+
 			if (arrclone[i - 1] == arrclone[i] - 1) {
 				ans++;
 				max = Math.max(ans, max);
+
+			} else if (arrclone[i] == arrclone[i - 1]) { // for duplicate elements
+				continue;
+
 			} else {
 				ans = 1;
 			}
@@ -55,17 +68,59 @@ public static int lengthOfLongestConsecutiveSequence2(int[] arr, int N) {
 		return max;
 	}
 
+	// approach 3, most optimal - using HashSet
+	// O(N) time | O(N) Space
+	public static int lengthOfLongestConsecutiveSequence3(int[] arr, int N) {
+		if (N == 0) {
+			return 0;
+		}
+		Set<Integer> myset = new HashSet<Integer>();
+		for (int num : arr) {
+			myset.add(num);
+		}
+
+		int maxlen = 1;
+		for (int n : arr) {
+			int currmax = 1;
+
+			if (myset.contains(n - 1)) // skip already searched sequence
+				continue;
+			else {
+				while (myset.contains(n + 1)) {
+					currmax++;
+					n++;
+				}
+			}
+			maxlen = Math.max(currmax, maxlen);
+		}
+
+		return maxlen;
+	}
+
 	public static void main(String[] args) {
 
-//		int[] arr = { 100, 4, 200, 1, 3, 2 }; 
-//		int[] arr = { 1, 5, 9, 13, 6, 12, 7, 11, 14, 15 };
-		int[] arr = { 9, 5, 4, 9, 10, 10, 6 };
-		int[] arr2 = { 0, 3, 7, 2, 5, 8, 4, 6, 0, 1 };
+		int[] arr = { 100, 4, 200, 1, 3, 2 };
+		int[] arr2 = { 1, 5, 9, 13, 6, 12, 7, 11, 14, 15 };
+		int[] arr3 = { 9, 5, 4, 9, 10, 10, 6 };
+		int[] arr4 = { 0, 3, 7, 2, 5, 8, 4, 6, 0, 1 };
+
+		System.out.println("brute force solution...");
 		System.out.println(lengthOfLongestConsecutiveSequence(arr, 7));
 		System.out.println(lengthOfLongestConsecutiveSequence(arr2, 9));
+		System.out.println(lengthOfLongestConsecutiveSequence(arr3, 7));
+		System.out.println(lengthOfLongestConsecutiveSequence(arr4, 9));
 
+		System.out.println("\ntest using sorting technique...");
 		System.out.println(lengthOfLongestConsecutiveSequence2(arr, 7));
 		System.out.println(lengthOfLongestConsecutiveSequence2(arr2, 9));
+		System.out.println(lengthOfLongestConsecutiveSequence2(arr3, 7));
+		System.out.println(lengthOfLongestConsecutiveSequence2(arr4, 9));
+
+		System.out.println("\ntest using hashset...");
+		System.out.println(lengthOfLongestConsecutiveSequence3(arr, 7));
+		System.out.println(lengthOfLongestConsecutiveSequence3(arr2, 9));
+		System.out.println(lengthOfLongestConsecutiveSequence3(arr3, 7));
+		System.out.println(lengthOfLongestConsecutiveSequence3(arr4, 9));
 
 	}
 }