Completed Pre Course 1

Exercise_1.java

@@ -7,29 +7,54 @@ class Stack {
 
     boolean isEmpty() 
     { 
-        //Write your code here 
+        //Write your code here
+        if(a.length==0)
+        {
+            return true;
+        }
+        return false;
     } 
 
     Stack() 
     { 
-        //Initialize your constructor 
+        //Initialize your constructor
+        top = -1;
     } 
 
     boolean push(int x) 
     { 
         //Check for stack Overflow
         //Write your code here
+        if(isEmpty())
+        {
+            top++;
+            a[top] = x;
+        }
+        a[++top]=x;
+        return true;
     } 
 
     int pop() 
-    { 
+    {
+        if(isEmpty())
+        {
+            System.out.println("Stack Underflow");
+            return 0;
+        }
+        return a[top--];
         //If empty return 0 and print " Stack Underflow"
         //Write your code here
+      //  return 1;
     } 
 
     int peek() 
     { 
         //Write your code here
+        if(isEmpty())
+        {
+            return -1;
+        }
+        return a[top];
     } 
 } 
 

Exercise_2.java

@@ -1,52 +1,76 @@
-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()); 
-    } 
-} 
+class StackAsLinkedList {
+
+        StackNode root;
+
+        static class StackNode {
+            int data;
+            StackNode next;
+
+            StackNode(int data) {
+                //Constructor here
+                this.data = data;
+            }
+        }
+
+
+        public boolean isEmpty() {
+            //Write your code here for the condition if stack is empty.
+            if (root == null) {
+                return true;
+            }
+            return false;
+        }
+
+        public void push(int data) {
+            //Write code to push data to the stack.
+            StackNode newNode = new StackNode(data);
+//            if (isEmpty()) {
+//                root = newNode;
+//            }
+            newNode.next = root;
+            root = newNode;
+            //   root.next = newNode;
+
+        }
+
+        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;
+            }
+            int data = root.data;
+            root = root.next;
+            return data;
+        }
+
+        public int peek() {
+            //Write code to just return the topmost element without removing it.
+            if (isEmpty()) {
+                System.out.println("Stack Underflow");
+                return 0;
+            }
+            int data = root.data;
+
+            return data;
+        }
+    }
+class Exercise_2 {
+
+    //Driver code
+    public static void main(String[] args) {
+
+        StackAsLinkedList sll = new StackAsLinkedList();
+
+        sll.push(10);
+        sll.push(20);
+        sll.push(30);
+        //  sll.pop();
+        System.out.println(sll.pop() + " popped from stack");
+
+        System.out.println("Top element is " + sll.peek());
+    }
+
+}