diff --git a/Exercise_1.java b/Exercise_1.java
index 314a3cb45..2c6ddacc8 100644
--- a/Exercise_1.java
+++ b/Exercise_1.java
@@ -1,46 +1,85 @@
-class Stack { 
+/*
+Space Complexity
+O(n) n is the number of elements in the stack
+
+Time Complexity
+isEmpty     O(1)
+push(int x) O(1)
+pop()       O(1)
+peek()      O(1)
+*/
+class StackAsArray {
     //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 a[] = new int[MAX]; // Maximum size of Stack 
-  
-    boolean isEmpty() 
-    { 
-        //Write your code here 
-    } 
-
-    Stack() 
-    { 
-        //Initialize your constructor 
-    } 
-  
-    boolean push(int x) 
-    { 
+    static final int MAX = 1000;
+    int top;
+    int a[] = new int[MAX]; // Maximum size of Stack
+
+    boolean isEmpty()
+    {
+        //Write your code here
+        if(top==-1){
+            System.out.println("Stack is Empty");
+            return true;
+        }
+        return false;
+    }
+
+    StackAsArray()
+    {
+        //Initialize your constructor
+        top = -1;
+    }
+
+    boolean push(int x)
+    {
         //Check for stack Overflow
         //Write your code here
-    } 
-  
-    int pop() 
-    { 
+        if(top==MAX-1){
+            System.out.println("Stack Overflow");
+            return false;
+        }
+
+        top=top+1;
+        a[top]=x;
+        System.out.println(x+" is pushed onto the stack");
+        return true;
+    }
+
+    int pop()
+    {
         //If empty return 0 and print " Stack Underflow"
         //Write your code here
-    } 
-  
-    int peek() 
-    { 
+        if(top==-1){
+            System.out.println("Stack Underflow");
+            return 0;
+        }
+
+        int value=a[top];
+        top=top-1;
+        return value;
+    }
+
+    int peek()
+    {
         //Write your code here
-    } 
-} 
-  
-// Driver code 
-class Main { 
-    public static void main(String args[]) 
-    { 
-        Stack s = new Stack(); 
-        s.push(10); 
-        s.push(20); 
-        s.push(30); 
-        System.out.println(s.pop() + " Popped from stack"); 
-    } 
+        if(top==-1){
+            System.out.println("Stack Underflow");
+            return 0;
+        }
+        return a[top];
+    }
+}
+
+// Driver code
+class Main {
+    public static void main(String args[])
+    {
+        // Stack
+        StackAsArray s = new StackAsArray();
+        s.push(10);
+        s.push(20);
+        s.push(30);
+        System.out.println(s.pop() + " Popped from stack");
+    }
 }
diff --git a/Exercise_2.java b/Exercise_2.java
index 5a9c4868c..eb2d301c8 100644
--- a/Exercise_2.java
+++ b/Exercise_2.java
@@ -1,52 +1,91 @@
-public class StackAsLinkedList { 
-  
-    StackNode root; 
-  
-    static class StackNode { 
-        int data; 
-        StackNode next; 
-  
-        StackNode(int data) 
-        { 
-            //Constructor here 
-        } 
-    } 
-    
-	
-    public boolean isEmpty() 
-    { 
-        //Write your code here for the condition if stack is empty. 
-    } 
-  
-    public void push(int data) 
-    { 
-        //Write code to push data to the stack. 
-    } 
-  
-    public int pop() 
-    { 	
+/*
+Space Complexity
+O(n) n is the number of elements in the stack
+
+Time Complexity
+isEmpty     O(1)
+push(int data) O(1)
+pop()       O(1)
+peek()      O(1)
+*/
+public class Exercise_2 {
+
+    StackNode root;
+
+    Exercise_2(){
+        root=null;
+    }
+
+    static class StackNode {
+        int data;
+        StackNode next;
+
+        StackNode(int data)
+        {
+            //Constructor here
+            this.data=data;
+            this.next=null;
+        }
+    }
+
+
+    public boolean isEmpty()
+    {
+        //Write your code here for the condition if stack is empty.
+        if(root==null){
+            System.out.println("Stack Underflow");
+            return true;
+        }
+        return false;
+    }
+
+    public void push(int data)
+    {
+        //Write code to push data to the stack.
+        StackNode newNode=new StackNode(data);
+        if(newNode==null){
+            System.out.println("Stack Overflow");
+            return;
+        }
+        newNode.next=root;
+        System.out.println(newNode.data+" is pushed onto the stack");
+        root=newNode;
+    }
+
+    public int pop()
+    {
 	//If Stack Empty Return 0 and print "Stack Underflow"
         //Write code to pop the topmost element of stack.
-	//Also return the popped element 
-    } 
-  
-    public int peek() 
-    { 
+	//Also return the popped element
+        if(isEmpty()){
+            return 0;
+        }
+        StackNode temp= root;
+        root=root.next;
+        return temp.data;
+    }
+
+    public int peek()
+    {
         //Write code to just return the topmost element without removing it.
-    } 
-  
+        if(isEmpty()){
+            return 0;
+        }
+        return root.data;
+    }
+
 	//Driver code
-    public static void main(String[] args) 
-    { 
-  
-        StackAsLinkedList sll = new StackAsLinkedList(); 
-  
-        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()); 
-    } 
-} 
+    public static void main(String[] args)
+    {
+
+        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 fb66d329d..0df3f700f 100644
--- a/Exercise_3.java
+++ b/Exercise_3.java
@@ -1,70 +1,94 @@
-import java.io.*; 
-  
-// Java program to implement 
-// a Singly Linked List 
-public class LinkedList { 
-  
-    Node head; // head of list 
-  
-    // Linked list Node. 
-    // This inner class is made static 
-    // so that main() can access it 
-    static class Node { 
-  
-        int data; 
-        Node next; 
-  
-        // Constructor 
-        Node(int d) 
-        { 
-            //Write your code here 
-        } 
-    } 
-  
-    // Method to insert a new node 
-    public static LinkedList insert(LinkedList list, int data) 
-    { 
-        // Create a new node with given data 
-   
-        // If the Linked List is empty, 
-        // then make the new node as head 
-        
-            // Else traverse till the last node 
-            // and insert the new_node there 
-
-            // Insert the new_node at last node 
-        // Return the list by head 
-        
-    } 
-  
-    // Method to print the LinkedList. 
-    public static void printList(LinkedList list) 
-    {  
-        // Traverse through the LinkedList 
-   
-            // Print the data at current node 
-       
-            // Go to next node 
-    } 
-   
-    // Driver code 
-    public static void main(String[] args) 
-    { 
+// Java program to implement
+// a Singly Linked List
+
+/*
+Space Complexity
+O(n) n is the number of elements in the linkedlist
+
+Time Complexity
+insert     O(n) n is the number of elements in the linkedlist to be traversed
+printList  O(n) n is the number of elements in the linkedlist
+*/
+public class Exercise_3 {
+
+    Node head; // head of list
+
+    // Linked list Node.
+    // This inner class is made static
+    // so that main() can access it
+    static class Node {
+
+        int data;
+        Node next;
+
+        // Constructor
+        Node(int d)
+        {
+            //Write your code here
+            data=d;
+            next=null;
+        }
+    }
+
+    // Method to insert a new node
+    public static Exercise_3 insert(Exercise_3 list, int data)
+    {
+        // Create a new node with given data
+        Node newNode=new Node(data);
+
+
+        // If the Linked List is empty,
+        // then make the new node as head
+        if(list.head==null){
+            list.head=newNode;
+        }
+        else{
+            // Else traverse till the last node
+            // and insert the new_node there
+            Node last=list.head;
+            while(last.next!=null){
+                last=last.next;
+            }
+
+            // Insert the new_node at last node
+            last.next=newNode;
+        }
+
+        // Return the list by head
+        return list;
+    }
+
+    // Method to print the LinkedList.
+    public static void printList(Exercise_3 list)
+    {
+        // Traverse through the LinkedList
+        Node node=list.head;
+        while(node!=null){
+            // Print the data at current node
+            System.out.println(node.data);
+            // Go to next node
+            node=node.next;
+        }
+    }
+
+    // Driver code
+    public static void main(String[] args)
+    {
         /* Start with the empty list. */
-        LinkedList list = new LinkedList(); 
-  
-        // 
-        // ******INSERTION****** 
-        // 
-  
-        // Insert the values 
-        list = insert(list, 1); 
-        list = insert(list, 2); 
-        list = insert(list, 3); 
-        list = insert(list, 4); 
-        list = insert(list, 5); 
-  
-        // Print the LinkedList 
-        printList(list); 
-    } 
+        Exercise_3 list = new Exercise_3();
+
+        //
+        // ******INSERTION******
+        //
+
+        // Insert the values
+        list = insert(list, 1);
+        list = insert(list, 2);
+        list = insert(list, 3);
+        list = insert(list, 4);
+        list = insert(list, 5);
+
+        // Print the LinkedList
+        printList(list);
+    }
 }
\ No newline at end of file