Completed PreCourse - 1

Exercise_1.java

@@ -1,36 +1,67 @@
-class Stack { 
+class Stack {
+    // Time Complexity :
+    // Space Complexity :
+    //Push - TC - O(1) SC - O(1)
+    //Peek - TC - O(1) SC - O(1)
+    //Pop - TC - O(1) SC - O(1)
+    // Did this code successfully run on Leetcode : not found on leetcode
+    // Any problem you faced while coding this : I set the top incorrectly first, but fixed it later
+
     //Please read sample.java file before starting.
   //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 
+    {
+        //Write your code here
+        return top == 0;
     } 
 
     Stack() 
-    { 
-        //Initialize your constructor 
+    {
+        //Initialize your constructor
+        top = 0;
     } 
 
     boolean push(int x) 
-    { 
+    {
+        if(top == MAX-1){
+            System.out.println("Stack overflow");
+            return false;
+        }
+        a[top] = x;
+        top++;
+        return true;
         //Check for stack Overflow
         //Write your code here
+
     } 
 
-    int pop() 
-    { 
+    int pop()
+    {
         //If empty return 0 and print " Stack Underflow"
         //Write your code here
+        if(top == 0){
+            System.out.println("Stack underflow");
+            return 0;
+        }
+        int tmp = a[top-1];
+        return tmp;
     } 
 
     int peek() 
-    { 
+    {
+        if(top == 0){
+            System.out.println("Stack underflow");
+            return 0;
+        }
+        int tmp = a[top-1];
+        return tmp;
         //Write your code here
-    } 
+    }
+
 } 
 
 // Driver code 

Exercise_2.java

@@ -1,4 +1,13 @@
-public class StackAsLinkedList { 
+// Time Complexity :
+// Space Complexity :
+//Push - TC - O(1) SC - O(1)
+//Peek - TC - O(1) SC - O(1)
+//Pop - TC - O(1) SC - O(1)
+// Did this code successfully run on Leetcode : not found on leetcode
+// Any problem you faced while coding this : Yes, figuring out the part on how to handle the root scenario. Also - adding to the start of the LL not the end
+
+
+class StackAsLinkedList {
 
     StackNode root; 
 
@@ -7,31 +16,49 @@ static class StackNode {
         StackNode next; 
 
         StackNode(int data) 
-        { 
+        {
+            this.data = data;
+            this.next = null;
             //Constructor here 
         } 
-    } 
+    }
 
 
     public boolean isEmpty() 
     { 
-        //Write your code here for the condition if stack is empty. 
+        //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. 
+    {
+        //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"
         //Write code to pop the topmost element of stack.
-	//Also return the popped element 
+	//Also return the popped element
+        if(root == null){
+            System.out.println("Stack Underflow");
+            return 0;
+        }
+        int tmp = root.data;
+        root = root.next;
+        return tmp;
     } 
 
     public int peek() 
-    { 
+    {
+        if(root == null){
+            System.out.println("Stack Underflow");
+            return 0;
+        }
+        return root.data;
         //Write code to just return the topmost element without removing it.
     } 
 

Exercise_3.java

@@ -1,10 +1,19 @@
-import java.io.*; 
+// Time Complexity :
+// Space Complexity :
+//N is the length of the linkedList at the point
+//Insert - TC - O(N) SC - O(N)
+//Print - TC - O(N) SC - O(N)
+// Did this code successfully run on Leetcode : didnt find the problem on leetcode
+// Any problem you faced while coding this : -
+
+
+import java.io.*;
 
 // Java program to implement 
 // a Singly Linked List 
-public class LinkedList { 
+class LinkedList {
 
-    Node head; // head of list 
+    static Node head; // head of list
 
     // Linked list Node. 
     // This inner class is made static 
@@ -16,14 +25,16 @@ static class Node {
 
         // Constructor 
         Node(int d) 
-        { 
+        {
+            this.data = d;
+            this.next = null;
             //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, 
@@ -33,8 +44,19 @@ public static LinkedList insert(LinkedList list, int data)
             // and insert the new_node there 
 
             // Insert the new_node at last node 
-        // Return the list by head 
+        // Return the list by head
 
+        Node newNode = new Node(data);
+        if(list.head == null){
+           list.head = newNode;
+        }else{
+            Node curr = list.head;
+            while(curr.next !=null ){
+                curr = curr.next;
+            }
+            curr.next = newNode;
+        }
+        return list;
     } 
 
     // Method to print the LinkedList. 
@@ -44,7 +66,12 @@ public static void printList(LinkedList list)
 
             // Print the data at current node 
 
-            // Go to next node 
+            // Go to next node
+        Node curr = head;
+        while(curr != null){
+            System.out.print(curr.data+" ");
+            curr = curr.next;
+        }
     } 
 
     // Driver code