PreCourse-1

Exercise_1.java

@@ -3,38 +3,59 @@ class Stack {
   //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 
+    int a[] = new int[MAX]; // Maximum size of Stack
+    int numElements=0;
 
     boolean isEmpty() 
     { 
-        //Write your code here 
+        //Write your code here
+    	return numElements==0;
     } 
 
     Stack() 
     { 
         //Initialize your constructor 
+    	this.top=0;
     } 
 
     boolean push(int x) 
     { 
         //Check for stack Overflow
+    	if (this.top >= this.a.length) {
+    		return false;
+    	}
         //Write your code here
+    	this.a[this.top++]=x;
+    	this.numElements++;
+    	System.out.println("Pushed " + x + " with top "+ this.top);
+    	return true;
     } 
 
     int pop() 
     { 
         //If empty return 0 and print " Stack Underflow"
+    	if (this.numElements==0) {
+    		System.out.println("Stack Underflow");
+    		return 0;
+    	}
         //Write your code here
+    	this.top--;
+    	int pop= this.a[this.top];
+    	this.a[this.top]=Integer.MIN_VALUE;
+    	this.numElements--;
+    	return pop;
+
     } 
 
     int peek() 
     { 
         //Write your code here
+    	return this.a[this.top];
     } 
 } 
 
 // Driver code 
-class Main { 
+class Exercise_1 { 
     public static void main(String args[]) 
     { 
         Stack s = new Stack(); 

Exercise_2.java

@@ -1,4 +1,21 @@
-public class StackAsLinkedList { 
+// StackAsLinkedList
+public class Exercise_2 { 
+	//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()); 
+    } 
+}
+class StackAsLinkedList { 
 
     StackNode root; 
 
@@ -9,44 +26,45 @@ static class StackNode {
         StackNode(int data) 
         { 
             //Constructor here 
+        	this.data=data;
         } 
     } 
 
 
     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 Stack Empty Return 0 and print "Stack Underflow"
         //Write code to pop the topmost element of stack.
-	//Also return the popped element 
+	    //Also return the popped element 
+    	if (isEmpty()) {
+    		System.out.println("Stack Underflow");
+    		return 0;
+    	}
+    	StackNode prevNode= root.next;
+    	int poppedValue= root.data;
+    	root = prevNode;
+    	return poppedValue;
     } 
 
     public int peek() 
     { 
         //Write code to just return the topmost element without removing it.
+    	int value = root.data;
+    	return value;
     } 
 
-	//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()); 
-    } 
 } 

Exercise_3.java

@@ -1,70 +1,92 @@
-import java.io.*; 
-  
+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) 
-    { 
-        /* 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); 
-    } 
+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
+			this.data = d;
+		}
+	}
+
+	// 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
+
+			// Insert the new_node at last node
+			Node current = list.head;
+			while (current.next != null) {
+				current = current.next;
+			}
+			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
+
+		// Print the data at current node
+
+		// Go to next node
+		Node current = list.head;
+
+		while (current != null) {
+			// Print the data at current node
+			System.out.print(current.data + " ");
+
+			// Go to next node
+			current = current.next;
+		}
+		System.out.println();
+	}
+
+}
+
+public class Exercise_3 {
+	// Driver code
+	public static void main(String[] args) {
+		/* Start with the empty list. */
+		LinkedList list = new LinkedList();
+
+		//
+		// ******INSERTION******
+		//
+
+		// Insert the values
+		list = list.insert(list, 1);
+		list = list.insert(list, 2);
+		list = list.insert(list, 3);
+		list = list.insert(list, 4);
+		list = list.insert(list, 5);
+
+		// Print the LinkedList
+		list.printList(list);
+	}
 }