diff --git a/Exercise_1.java b/Exercise_1.java
index 314a3cb45..fbb51546f 100644
--- a/Exercise_1.java
+++ b/Exercise_1.java
@@ -1,6 +1,12 @@
 class Stack { 
     //Please read sample.java file before starting.
   //Kindly include Time and Space complexity at top of each file
+  //timecomplexity
+  //isEmpty() ->O(1)
+  //push()->O(1)
+  //pop()->O(1)
+  //peek()->O(1)
+  //Spece Complexity: O(MAX)
     static final int MAX = 1000; 
     int top; 
     int a[] = new int[MAX]; // Maximum size of Stack 
@@ -8,28 +14,49 @@ class Stack {
     boolean isEmpty() 
     { 
         //Write your code here 
+        return(top < 0);
     } 
 
     Stack() 
     { 
         //Initialize your constructor 
+        top = -1; //stack is empty
     } 
   
     boolean push(int x) 
     { 
         //Check for stack Overflow
         //Write your code here
+        //push : first increment top then store value
+        if(top >= MAX-1){
+            System.out.println("Stack is overflow");
+            return false;
+        }else{
+            a[++top] = x;
+            return true;
+        }
     } 
   
     int pop() 
     { 
         //If empty return 0 and print " Stack Underflow"
         //Write your code here
+        if(isEmpty()){
+            System.out.println("Stack underflow");
+            return 0;
+        }else{
+            return a[top--];
+        }
     } 
   
     int peek() 
     { 
-        //Write your code here
+        if(isEmpty()){
+            System.out.println("Stack underflow");
+            return 0;
+        }else{
+            return a[top];
+        }
     } 
 } 
   
diff --git a/Exercise_2$StackNode.class b/Exercise_2$StackNode.class
new file mode 100644
index 000000000..0760c5ea3
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diff --git a/Exercise_2.class b/Exercise_2.class
new file mode 100644
index 000000000..709d9e5d2
Binary files /dev/null and b/Exercise_2.class differ
diff --git a/Exercise_2.java b/Exercise_2.java
index 5a9c4868c..08e88de51 100644
--- a/Exercise_2.java
+++ b/Exercise_2.java
@@ -1,52 +1,64 @@
-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() 
-    { 	
-	//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() 
-    { 
-        //Write code to just return the topmost element without removing it.
-    } 
-  
-	//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 class Exercise_2 {
+
+    // Time Complexity:
+    // isEmpty() -> O(1)
+    // push()    -> O(1)
+    // pop()     -> O(1)
+    // peek()    -> O(1)
+    //
+    // Space Complexity:
+    // O(n)
+
+    StackNode root;
+
+    static class StackNode {
+        int data;
+        StackNode next;
+
+        StackNode(int data) {
+            this.data = data;
+            this.next = null;
+        }
+    }
+
+    public boolean isEmpty() {
+        return root == null;
+    }
+
+    public void push(int data) {
+        StackNode newNode = new StackNode(data);
+        newNode.next = root;
+        root = newNode;
+    }
+
+    public int pop() {
+        if (isEmpty()) {
+            System.out.println("Stack Underflow");
+            return 0;
+        }
+
+        int popped = root.data;
+        root = root.next;
+        return popped;
+    }
+
+    public int peek() {
+        if (isEmpty()) {
+            System.out.println("Stack Underflow");
+            return 0;
+        }
+
+        return root.data;
+    }
+
+    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());
+    }
+}
\ No newline at end of file
diff --git a/Exercise_3$Node.class b/Exercise_3$Node.class
new file mode 100644
index 000000000..7b630f685
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diff --git a/Exercise_3.class b/Exercise_3.class
new file mode 100644
index 000000000..500bada30
Binary files /dev/null and b/Exercise_3.class differ
diff --git a/Exercise_3.java b/Exercise_3.java
index fb66d329d..6fd518b64 100644
--- a/Exercise_3.java
+++ b/Exercise_3.java
@@ -1,70 +1,63 @@
-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 
+import java.io.*;
 
-            // 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) 
-    { 
-        /* 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); 
-    } 
+// Time Complexity:
+// insert()    -> O(n)
+// printList() -> O(n)
+//
+// Space Complexity:
+// O(n)
+
+public class Exercise_3 {
+
+    Node head; // head of list
+
+    static class Node {
+        int data;
+        Node next;
+
+        Node(int d) {
+            this.data = d;
+            this.next = null;
+        }
+    }
+
+    // Method to insert a new node
+    public static Exercise_3 insert(Exercise_3 list, int data) {
+        Node new_node = new Node(data);
+
+        if (list.head == null) {
+            list.head = new_node;
+        } else {
+            Node last = list.head;
+            while (last.next != null) {
+                last = last.next;
+            }
+            last.next = new_node;
+        }
+
+        return list;
+    }
+
+    // Method to print the LinkedList
+    public static void printList(Exercise_3 list) {
+        Node currNode = list.head;
+
+        while (currNode != null) {
+            System.out.print(currNode.data + " ");
+            currNode = currNode.next;
+        }
+    }
+
+    // Driver code
+    public static void main(String[] args) {
+        Exercise_3 list = new Exercise_3();
+
+        list = insert(list, 1);
+        list = insert(list, 2);
+        list = insert(list, 3);
+        list = insert(list, 4);
+        list = insert(list, 5);
+
+        printList(list);
+    }
 }
\ No newline at end of file
diff --git a/Main.class b/Main.class
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index 000000000..838c9b12b
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diff --git a/Stack.class b/Stack.class
new file mode 100644
index 000000000..22ffb777e
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