Completed Precourse 1

Exercise_1.java

@@ -1,46 +1,68 @@
 class Stack { 
     //Please read sample.java file before starting.
-  //Kindly include Time and Space complexity at top of each file
+    //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 
+    boolean isEmpty() { 
+        // Write your code here 
+        return (top == -1); 
     } 
 
-    Stack() 
-    { 
-        //Initialize your constructor 
+    Stack() { 
+        // Initialize your constructor 
+        top = -1;
     } 
 
-    boolean push(int x) 
-    { 
-        //Check for stack Overflow
-        //Write your code here
+    boolean push(int x) { 
+        // Check for stack Overflow
+        // Write your code here
+        if (top == MAX - 1) {
+            System.out.println("Overflow");
+            return false;
+        }
+        a[++top] = x;//element is pushed to array for stack
+        return true;
     } 
 
-    int pop() 
-    { 
-        //If empty return 0 and print " Stack Underflow"
-        //Write your code here
+    int pop() { 
+        // If empty return 0 and print " Stack Underflow"
+        // Write your code here
+        if (top == -1) {
+            System.out.println("Underflow");
+            return 0;
+        }
+        return a[top--];
     } 
 
-    int peek() 
-    { 
-        //Write your code here
+    int peek() { 
+        // Write your code here
+        if (top == -1) {
+            System.out.println("Underflow");
+            return 0;
+        }
+        return a[top];
+    } 
+
+    void print() { 
+        for (int i = top; i > -1; i--) { 
+            System.out.print(" " + a[i]); 
+        }
+        System.out.println(); // Add a newline for better output readability
     } 
 } 
 
 // Driver code 
 class Main { 
-    public static void main(String args[]) 
-    { 
+    public static void main(String args[]) { 
         Stack s = new Stack(); 
         s.push(10); 
         s.push(20); 
         s.push(30); 
+        s.print();
         System.out.println(s.pop() + " Popped from stack"); 
+        s.print();
     } 
 }

Exercise_2.java

@@ -1,4 +1,4 @@
-public class StackAsLinkedList { 
+class StackAsLinkedList { 
 
     StackNode root; 
 
@@ -9,30 +9,57 @@ 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) 
     { 
+        StackNode new_node = new StackNode(data);
         //Write code to push data to the stack. 
+        if(root==null){
+            root=new_node;
+        }
+        else{
+            new_node.next=root;//new element is inserted in the top of the stack
+            root=new_node;
+        }
     } 
 
     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 
+
+    if(isEmpty()){
+        System.out.println("Stack Underflow");
+        return 0;
+    }
+    else{
+        int popped = root.data; // getting the popped value
+        root=root.next;
+        return popped;
+    }
     } 
 
     public int peek() 
     { 
         //Write code to just return the topmost element without removing it.
+        if(isEmpty()){
+            System.out.println("Stack is Empty");
+            return 0;
+        }
+        return root.data; // return the top data
     } 
 
 	//Driver code

Exercise_3.java

@@ -2,7 +2,7 @@
 
 // Java program to implement 
 // a Singly Linked List 
-public class LinkedList { 
+class LinkedList { 
 
     Node head; // head of list 
 
@@ -18,13 +18,16 @@ 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 new_node = new Node(data);
 
         // If the Linked List is empty, 
         // then make the new node as head 
@@ -34,7 +37,18 @@ public static LinkedList insert(LinkedList list, int data)
 
             // Insert the new_node at last node 
         // Return the list by head 
-
+
+        if(list.head==null){
+            list.head=new_node;
+        }
+        else{
+            Node temp=list.head;
+            while(temp.next!=null){
+                temp=temp.next;
+            }
+            temp.next=new_node; // node inserted at the end of the lisr
+        }
+        return list; 
     } 
 
     // Method to print the LinkedList. 
@@ -45,6 +59,11 @@ public static void printList(LinkedList list)
             // Print the data at current node 
 
             // Go to next node 
+            Node temp=list.head;
+            while(temp!=null){
+                System.out.println(temp.data); // prints each value in the lisr
+                temp=temp.next;
+            }
     } 
 
     // Driver code