Precourse 1 completed

Exercise_1.java

@@ -7,29 +7,49 @@ class Stack {
 
     boolean isEmpty() 
     { 
-        //Write your code here 
+       return top < 0;
     } 
 
     Stack() 
     { 
-        //Initialize your constructor 
+        this.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
+        {
+            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 -1;
+        }
+      else
+        {
+            return a[top--];
+        }
     } 
 
     int peek() 
-    { 
-        //Write your code here
+    {
+        if (isEmpty()) {
+            System.out.println("Stack Underflow");
+            return 0;
+        } else {
+            return a[top];
+        }
     } 
 } 
 
@@ -40,7 +60,8 @@ public static void main(String args[])
         Stack s = new Stack(); 
         s.push(10); 
         s.push(20); 
-        s.push(30); 
-        System.out.println(s.pop() + " Popped from stack"); 
+        s.push(30);
+        System.out.println(s.pop() + " Popped from stack");
+        System.out.println("Top element is " + s.peek());
     } 
 }

Exercise_3.java → LinkedList.java

@@ -1,6 +1,4 @@
-import java.io.*; 
-
-// Java program to implement 
+// Java program to implement
 // a Singly Linked List 
 public class LinkedList { 
 
@@ -17,25 +15,31 @@ static class Node {
         // Constructor 
         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 
-
-        // 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 
-
-    } 
+        Node newNode = new Node(data);
+
+        if(list.head == null)
+           list.head = newNode;
+        else
+        {
+            Node temp = list.head;
+            while (temp.next != null)
+            {
+                temp = temp.next;
+            }
+            temp.next = newNode;
+        }
+
+        return list;
+    }
 
     // Method to print the LinkedList. 
     public static void printList(LinkedList list) 
@@ -44,7 +48,13 @@ public static void printList(LinkedList list)
 
             // Print the data at current node 
 
-            // Go to next node 
+            // Go to next node
+        Node temp = list.head;
+        while(temp != null)
+        {
+            System.out.print(temp.data + " ");
+            temp = temp.next;
+        }
     } 
 
     // Driver code 

PreCourse-1.iml

@@ -0,0 +1,11 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<module type="JAVA_MODULE" version="4">
+  <component name="NewModuleRootManager" inherit-compiler-output="true">
+    <exclude-output />
+    <content url="file://$MODULE_DIR$">
+      <sourceFolder url="file://$MODULE_DIR$" isTestSource="false" />
+    </content>
+    <orderEntry type="inheritedJdk" />
+    <orderEntry type="sourceFolder" forTests="false" />
+  </component>
+</module>

StackAsLinkedList.java

@@ -0,0 +1,75 @@
+public class StackAsLinkedList {
+    // Time: push/pop/peek/isEmpty are all O(1).
+    // Space: O(n) for n nodes.
+
+    StackNode root; 
+
+    static class StackNode { 
+        int data; 
+        StackNode next; 
+
+        StackNode(int data) 
+        { 
+            this.data = data;
+            this.next = null;
+        } 
+    } 
+
+
+    public boolean isEmpty() 
+    {
+        return root == null;
+    }
+
+    public void push(int data) 
+    {
+        // Create a new node
+        StackNode newNode = new StackNode(data);
+        // New node points to current top
+        newNode.next = root;
+        // New node becomes the new top
+        root = newNode;
+    } 
+
+    public int pop() 
+    { 	
+
+        if (isEmpty())
+        {
+            System.out.println("Stack Underflow");
+            return 0;
+        }
+        else
+        {
+            int popData = root.data;
+            root = root.next;
+            return popData;
+        }
+    }
+
+    public int peek() 
+    {
+        if (isEmpty())
+        {
+            System.out.println("Stack Underflow");
+            return 0;
+        }
+        else
+        return root.data;
+    }
+
+	//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()); 
+    } 
+}