Pre course 1 submission

Exercise_1.java

@@ -1,35 +1,54 @@
 class Stack { 
     //Please read sample.java file before starting.
   //Kindly include Time and Space complexity at top of each file
+
+// Time Complexity :O(1) for all operations
+// Space Complexity : O(n) for storing elements in array
+// Did this code successfully run on Leetcode : NA
+
     static final int MAX = 1000; 
     int top; 
     int a[] = new int[MAX]; // Maximum size of Stack 
 
     boolean isEmpty() 
     { 
-        //Write your code here 
+        return top == -1;
     } 
 
     Stack() 
     { 
         //Initialize your constructor 
+        this.top = -1;
     } 
 
     boolean push(int x) 
     { 
-        //Check for stack Overflow
-        //Write your code here
+        if(top == a.length -1){
+            return false;
+        }
+        else{
+            a[++top] = x;
+            return true;
+        }
+
     } 
 
     int pop() 
     { 
         //If empty return 0 and print " Stack Underflow"
+        if(top == -1){
+            System.out.println("Stack Underflow");
+            return 0;
+        }
+         return a[top--];
         //Write your code here
     } 
 
     int peek() 
     { 
         //Write your code here
+        if(top == -1) return -1;
+        return a[top];
     } 
 } 
 

Exercise_2.java

@@ -1,4 +1,7 @@
-public class StackAsLinkedList { 
+//TC - O(1) for all operations
+//SC - O(n) for storing elements in linked list
+
+public class Exercise_2 { 
 
     StackNode root; 
 
@@ -8,38 +11,54 @@ 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(root == null){
+        System.out.println("Stack Underflow");
+        return 0;
+    }
         //Write code to pop the topmost element of stack.
-	//Also return the popped element 
+	    //Also return the popped element 
+        int val = root.data;
+        root = root.next;
+        return val;
     } 
 
     public int peek() 
     { 
         //Write code to just return the topmost element without removing it.
+        if(root == null){
+            System.out.println("stack is empty");
+            return -1;
+        }
+        return root.data;
     } 
 
 	//Driver code
     public static void main(String[] args) 
     { 
 
-        StackAsLinkedList sll = new StackAsLinkedList(); 
+        Exercise_2 sll = new Exercise_2(); 
 
         sll.push(10); 
         sll.push(20); 

Exercise_3.java

@@ -2,7 +2,7 @@
 
 // Java program to implement 
 // a Singly Linked List 
-public class LinkedList { 
+public class Exercise_3 { 
 
     Node head; // head of list 
 
@@ -12,17 +12,21 @@ public class LinkedList {
     static class Node { 
 
         int data; 
-        Node next; 
+        Node next;
+
 
         // Constructor 
-        Node(int d) 
+        Node(int data) 
         { 
             //Write your code here 
+            this.data = data;
+            this.next = null;
+
         } 
     } 
 
     // Method to insert a new node 
-    public static LinkedList insert(LinkedList list, int data) 
+    public static Exercise_3 insert(Exercise_3 list, int data) 
     { 
         // Create a new node with given data 
 
@@ -34,29 +38,47 @@ public static LinkedList insert(LinkedList list, int data)
 
             // Insert the new_node at last node 
         // Return the list by head 
+        Node newNode = new Node(data);
+        //LL is empty
+        if(list.head == null){
+            list.head = newNode;
+            return list;
+        }
+
+        // LL is not empty so traverse till last and then insert the element
+        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) 
+    public static void printList(Exercise_3 list) 
     {  
         // Traverse through the LinkedList 
-
             // 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;
+            } 
+            System.out.println("null");
     } 
 
     // Driver code 
     public static void main(String[] args) 
     { 
         /* Start with the empty list. */
-        LinkedList list = new LinkedList(); 
+        Exercise_3 list = new Exercise_3(); 
 
         // 
         // ******INSERTION****** 
-        // 
-
+
         // Insert the values 
         list = insert(list, 1); 
         list = insert(list, 2);