Precourse1

Exercise_1.java

@@ -1,44 +1,80 @@
+// Time Complexity:
+//Space Complexity:
+
+
 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 
+        //Check top
+    	if (top==-1)
+    	{
+    		return true;
+    	}
+    	else
+    		return false;
     } 
 
     Stack() 
     { 
-        //Initialize your constructor 
+        //Intialize top
+    	top = -1;
     } 
 
     boolean push(int x) 
     { 
         //Check for stack Overflow
-        //Write your code here
+    	if (top>=MAX-1)
+    	{
+    		System.out.println("Stack Overflow");
+    		return false;
+    	}
+    	else{
+    	    top++;
+    		a[top] = x;
+    	}
+    // 	System.out.println(a);
+    		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 is empty");
+    	}
+    	return a[top];
     } 
 } 
 
 // Driver code 
-class Main { 
+public class Main { 
     public static void main(String args[]) 
     { 
         Stack s = new Stack(); 
-        s.push(10); 
+        s.push(10);
+        // System.out.println(s.a);
         s.push(20); 
         s.push(30); 
         System.out.println(s.pop() + " Popped from stack"); 

Exercise_2.java

@@ -1,52 +1,84 @@
-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 StackAsLinkedList {
+
+    StackNode root;
+
+    static class StackNode {
+        int data;
+        StackNode next;
+        StackNode previous;
+    }
+
+    void StackAsLinkedList() {
+        this.root = null;
+    }
+
+    public boolean isEmpty() {
+        if (root == null) {
+            return true;
+        } else
+            return false;
+    }
+
+    public void push(int data) {
+        StackNode temp = new StackNode();
+        if (temp == null) {
+            System.out.print("\nHeap Overflow");
+        }
+        if (root==null){
+            // System.out.println("In if");
+            // root=new StackNode();
+            temp.data=data;
+            temp.next=null;
+            temp.previous=null;
+            root=temp;
+        }
+        else{
+            // System.out.println("In else");
+            temp.data = data;
+            temp.previous=root;
+            root.next = temp;
+            temp.next=null;
+            root = temp;
+        }
+
+    }
+
+    public int pop() {
+        int pop_data;
+        // If Stack Empty Return 0 and print "Stack Underflow"
+        // Write code to pop the topmost element of stack.
+        // Also return the popped element
+        if (root == null) {
+            System.out.print("\nStack Underflow");
+            return 0;
+        }
+        pop_data = root.data;
+        // update the top pointer to point to the next node
+        root = (root).previous;
+        return pop_data;
+    }
+
+    public int peek() {
+        if (!isEmpty()) {
+            return root.data;
+        } else {
+            System.out.println("Stack is empty");
+            return -1;
+        }
+    }
+
+    // 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 +1,71 @@
-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 
+public class StackAsSinglyLinkedList {
 
-            // 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); 
-    } 
-}
+    StackNode root;
+
+    static class StackNode {
+        int data;
+        StackNode next;
+    }
+
+    void StackAsLinkedList() {
+        this.root = null;
+    }
+
+    public boolean isEmpty() {
+        if (root == null) {
+            return true;
+        } else
+            return false;
+    }
+
+    public void push(int data) {
+        StackNode temp = new StackNode();
+        if (temp == null) {
+            System.out.print("\nHeap Overflow");
+        }
+
+        temp.data = data;
+        temp.next = root;
+        root = temp;
+
+    }
+
+    public int pop() {
+        int pop_data;
+        // If Stack Empty Return 0 and print "Stack Underflow"
+        // Write code to pop the topmost element of stack.
+        // Also return the popped element
+        if (root == null) {
+            System.out.print("\nStack Underflow");
+            return 0;
+        }
+        pop_data = root.data;
+        // update the top pointer to point to the next node
+        root = (root).next;
+        return pop_data;
+    }
+
+    public int peek() {
+        if (!isEmpty()) {
+            return root.data;
+        } else {
+            System.out.println("Stack is empty");
+            return -1;
+        }
+    }
+
+    // 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());
+    }
+
+}