done Pre course exercise - 1

Exercise_1.java

@@ -5,31 +5,55 @@ class Stack {
     int top; 
     int a[] = new int[MAX]; // Maximum size of Stack 
 
+    //Time complexxity : O(1) for push, pop and peek operations
+    //Space complexity : O(n) where n is the number of elements in the stack
+
     boolean isEmpty() 
     { 
-        //Write your code here 
+        return top < 0; 
     } 
 
     Stack() 
     { 
-        //Initialize your constructor 
-    } 
+       top=-1; //Initialize your constructor 
+    }
 
     boolean push(int x) 
     { 
         //Check for stack Overflow
-        //Write your code here
+        if (top >= MAX - 1) {
+            System.out.println("Stack 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];
+        }
     } 
 } 
 

Exercise_2.java

@@ -1,4 +1,6 @@
-public class StackAsLinkedList { 
+//Time Complexity : O(1) for push, pop and peek operations
+//Space Complexity : O(n) 
+public class Exercise_2 { 
 
     StackNode root; 
 
@@ -8,38 +10,55 @@ static class StackNode {
 
         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. 
+        return root == null;
     } 
 
     public void push(int data) 
     { 
         //Write code to push data to the stack. 
+        StackNode newNode = new StackNode(data);
+        newNode.next = root;
+        root = newNode;
     } 
 
     public int pop() 
     { 	
 	//If Stack Empty Return 0 and print "Stack Underflow"
+        if (isEmpty()) {
+            System.out.println("Stack Underflow");
+            return 0;
+        }
+        int poppedData = root.data;
+        root = root.next;
+        return poppedData;
         //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.
+         //Write code to just return the topmost element without removing it.
+        if (isEmpty()) {
+            System.out.println("Stack Underflow");
+            return 0;
+        }
+        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); 

Exercise_3.java

@@ -1,11 +1,13 @@
+//Time Complexity : O(n) 
+//Space Complexity : O(1)
 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 
@@ -18,34 +20,50 @@ static class Node {
         Node(int d) 
         { 
             //Write your code here 
+            this.data = d;
+            this.next = null;
         } 
     } 
 
     // Method to insert a new node 
     public static LinkedList insert(LinkedList 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 current = list.head;
+            while (current.next != null) {
+                current = current.next;
+            }
             // Insert the new_node at last node 
+                 current.next = newNode;
+
+        }
         // Return the list by head 
-
+        return list;
     } 
 
     // Method to print the LinkedList. 
     public static void printList(LinkedList list) 
     {  
         // Traverse through the LinkedList 
-
+        Node current = list.head;
+        while (current != null) {
+
             // Print the data at current node 
-
+       System.out.println(current.data);
             // Go to next node 
+            current = current.next;
     } 
+    System.out.println();
+    }
 
     // Driver code 
     public static void main(String[] args)