chore(java): add longest consecutive subsequence (#356)

algorithms/Java/README.md

@@ -7,6 +7,7 @@
 3. [Left Rotation](arrays/left-rotation.java)
 4. [Unique Digits of Large Number](arrays/unique-digits-of-large-number.java)
 5. [Majority Element](arrays/majority-element.java)
+6. [Longest Consecutive Subsequence](arrays/longest-consecutive-subsequence.java)
 
 ## Graphs
 
@@ -22,6 +23,7 @@
 6. [Fold Linked List](linked-lists/fold-linked-list.java)
 
 ## Maths
+
 1. [Factorial](Maths/factorial_using_big_integer.java)
 
 ## Queues

algorithms/Java/arrays/longest-consecutive-subsequence.java

@@ -0,0 +1,87 @@
+/* Given an array of positive integers. Find the length of the longest sub-sequence such that elements in the subsequence are consecutive integers, 
+the consecutive numbers can be in any order. */
+
+/* Time Complexity: O(n) 
+   Space Complexity: O(n) */
+
+import java.math.*;
+import java.util.*;
+import java.io.*;
+
+class Driverclass {
+    static class FastReader{ 
+        BufferedReader br; 
+        StringTokenizer st; 
+  
+        public FastReader(){ 
+            br = new BufferedReader(new InputStreamReader(System.in)); 
+        } 
+  
+        String next(){ 
+            while (st == null || !st.hasMoreElements()){ 
+                try{ st = new StringTokenizer(br.readLine()); } catch (IOException  e){ e.printStackTrace(); } 
+            } 
+            return st.nextToken(); 
+        } 
+  
+        String nextLine(){ 
+            String str = ""; 
+            try{ str = br.readLine(); } catch (IOException e) { e.printStackTrace(); } 
+            return str; 
+        } 
+
+        Integer nextInt(){
+            return Integer.parseInt(next());
+        }
+    }
+    
+	public static void main(String args[]) {
+		FastReader sc = new FastReader();
+		PrintWriter out = new PrintWriter(System.out);
+		int t = sc.nextInt();
+		
+		while(t>0) {
+			int n = sc.nextInt();
+			int a[] = new int[n];
+			
+			for(int i=0; i<n; i++)
+				a[i] = sc.nextInt();
+		    out.println(new Solution().findLongestConseqSubseq(a, n));
+		    t--;
+		}
+		out.flush();
+	}
+}
+
+class Solution {   
+    // arr[] : the input array
+    // N : size of the array arr[]
+	static int findLongestConseqSubseq(int arr[], int n){
+	   Set<Integer> set = new HashSet<>();
+	   int ans=0;
+	   for(int num: arr)
+	       set.add(num);
+	   for(int i=0; i<n; i++){
+	       if(!set.contains(arr[i]-1)){
+	           int j = arr[i];
+	           while(set.contains(j))
+	               j++;
+	           ans = Math.max(ans, j-arr[i]);
+	       }
+	   }
+	   return ans;
+	}
+}
+
+/* 
+Input:
+N = 7
+a[] = {2,6,1,9,4,5,3}
+Output:
+6
+Explanation:
+The consecutive numbers here
+are 1, 2, 3, 4, 5, 6. These 6 
+numbers form the longest consecutive
+subsquence.
+ */