diff --git a/Exercise_1.class b/Exercise_1.class
new file mode 100644
index 00000000..adb44505
Binary files /dev/null and b/Exercise_1.class differ
diff --git a/Exercise_1.java b/Exercise_1.java
index 314a3cb4..6ab17598 100644
--- a/Exercise_1.java
+++ b/Exercise_1.java
@@ -1,46 +1,74 @@
+//Space complexity O(N)
+//Time complexity O(1)
 class Stack { 
     //Please read sample.java file before starting.
   //Kindly include Time and Space complexity at top of each file
     static final int MAX = 1000; 
-    int top; 
+    int top =-1; 
     int a[] = new int[MAX]; // Maximum size of Stack 
   
     boolean isEmpty() 
     { 
         //Write your code here 
+        if(top == -1) {
+             return true;
+        }
+        return false;
     } 
 
     Stack() 
     { 
         //Initialize your constructor 
     } 
-  
     boolean push(int x) 
     { 
         //Check for stack Overflow
+        if(top ==  MAX) {
+            System.out.println("Stack Overflow");
+            return false;
+        }
         //Write your code here
+        else {
+            a[++top] = x;
+            return true;
+        }
+        
     } 
   
     int pop() 
     { 
         //If empty return 0 and print " Stack Underflow"
+        if(top == -1) {
+            System.out.println( "Stack Underflow");
+            return 0;
+        }
         //Write your code here
+        else {
+            int temp = a[top];
+            top--;
+            return temp;
+        }
+        
     } 
   
     int peek() 
     { 
         //Write your code here
+        return a[top];
     } 
+
 } 
-  
+
+
+
 // Driver code 
-class Main { 
+class Exercise_1 { 
     public static void main(String args[]) 
     { 
         Stack s = new Stack(); 
         s.push(10); 
         s.push(20); 
-        s.push(30); 
+        s.push(30);    
         System.out.println(s.pop() + " Popped from stack"); 
     } 
 }
diff --git a/Exercise_2.java b/Exercise_2.java
index 5a9c4868..80e8f855 100644
--- a/Exercise_2.java
+++ b/Exercise_2.java
@@ -1,14 +1,19 @@
-public class StackAsLinkedList { 
+//TimeCOmplexity -> 0(1)
+//Space Complexity -> O(1)
+public class Exercise_2 { 
   
-    StackNode root; 
+    
+    StackNode root = null; 
   
     static class StackNode { 
         int data; 
         StackNode next; 
   
-        StackNode(int data) 
+        StackNode(int d) 
         { 
             //Constructor here 
+            data = d;
+            next = null;
         } 
     } 
     
@@ -16,35 +21,58 @@ static class StackNode {
     public boolean isEmpty() 
     { 
         //Write your code here for the condition if stack is empty. 
+        if(root == null) {
+            return true;
+        }
+        return false;
     } 
   
     public void push(int data) 
     { 
         //Write code to push data to the stack. 
+        StackNode node = new StackNode(data);
+        node.next = null;
+        if(root == null) {
+            root = node;
+        }else {
+           node.next = root;
+           root = node;
+        }
+        
     } 
   
     public int pop() 
     { 	
 	//If Stack Empty Return 0 and print "Stack Underflow"
-        //Write code to pop the topmost element of stack.
+    if(root == null) {
+        System.out.println("Stack Underflow");
+        return 0;
+    }
+    //Write code to pop the topmost element of stack.
 	//Also return the popped element 
+    int element = root.data;
+    root = root.next;
+    return element;
+   
     } 
   
     public int peek() 
     { 
         //Write code to just return the topmost element without removing it.
+        return root.data;
     } 
+
   
 	//Driver code
     public static void main(String[] args) 
     { 
   
-        StackAsLinkedList sll = new StackAsLinkedList(); 
+        Exercise_2 sll = new Exercise_2(); 
   
         sll.push(10); 
         sll.push(20); 
         sll.push(30); 
-  
+       
         System.out.println(sll.pop() + " popped from stack"); 
   
         System.out.println("Top element is " + sll.peek()); 
diff --git a/Exercise_3.java b/Exercise_3.java
index fb66d329..fe337d34 100644
--- a/Exercise_3.java
+++ b/Exercise_3.java
@@ -1,8 +1,9 @@
 import java.io.*; 
-  
+ //Time Complexity  - > O(N)
+ //Space Complexity  - > O(N)
 // Java program to implement 
 // a Singly Linked List 
-public class LinkedList { 
+public class Exercise_3 { 
   
     Node head; // head of list 
   
@@ -18,40 +19,62 @@ static class Node {
         Node(int d) 
         { 
             //Write your code here 
+            data = d;
+            next = null;
         } 
     } 
   
     // Method to insert a new node 
-    public static LinkedList insert(LinkedList list, int data) 
+    public static Exercise_3 insert(Exercise_3 list, int data) 
     { 
+
         // Create a new node with given data 
-   
-        // If the Linked List is empty, 
-        // then make the new node as head 
-        
+        Node n = new Node(data);
+        n.next = null;
+        // If the Linked List is empty,
+        if(list.head == null) {
+            // then make the new node as head 
+            list.head = n;
+        }else {
             // Else traverse till the last node 
             // and insert the new_node there 
+            // Insert the new_node at last node
+           Node temp = list.head;
+           while (temp.next != null) {
+            temp = temp.next;
+           }
+           temp.next = n;
+        }
+        
+       
+        
+            
 
-            // Insert the new_node at last node 
+           
         // Return the list by head 
-        
+        return list;
     } 
   
     // Method to print the LinkedList. 
-    public static void printList(LinkedList list) 
+    public static void printList(Exercise_3 list) 
     {  
+        Node temp = list.head;
         // Traverse through the LinkedList 
-   
+        while (temp.next != null) {
             // Print the data at current node 
-       
-            // Go to next node 
+            System.out.println(temp.data);
+            // Go to next node
+            temp = temp.next;
+        }
+        System.out.println(temp.data);
+ 
     } 
    
     // Driver code 
     public static void main(String[] args) 
     { 
         /* Start with the empty list. */
-        LinkedList list = new LinkedList(); 
+        Exercise_3 list = new Exercise_3(); 
   
         // 
         // ******INSERTION******