precourse-1 solution

Exercise_1.java

@@ -1,3 +1,6 @@
+// Time Complexity : O(1) for push, pop, peek, and isEmpty
+// Space Complexity : O(N) where N is the MAX size of the stack
+
 class Stack { 
     //Please read sample.java file before starting.
   //Kindly include Time and Space complexity at top of each file
@@ -7,29 +10,48 @@ class Stack {
 
     boolean isEmpty() 
     { 
-        //Write your code here 
+        return top < 0;
     } 
 
     Stack() 
     { 
-        //Initialize your constructor 
+        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;
+            System.out.println(x + " pushed into stack");
+            return true;
+        }
     } 
 
     int pop() 
     { 
         //If empty return 0 and print " Stack Underflow"
-        //Write your code here
+        if (top < 0) {
+            System.out.println("Stack Underflow");
+            return 0;
+        } else {
+            int x = a[top--];
+            return x;
+        }
     } 
 
     int peek() 
     { 
-        //Write your code here
+        if (top < 0) {
+            System.out.println("Stack Underflow");
+            return 0;
+        } else {
+            int x = a[top];
+            return x;
+        }
     } 
 } 
 

Exercise_2.java

@@ -1,4 +1,4 @@
-public class StackAsLinkedList { 
+class StackAsLinkedList { 
 
     StackNode root; 
 
@@ -8,31 +8,52 @@ static class StackNode {
 
         StackNode(int data) 
         { 
-            //Constructor here 
+            //Constructor here
+        	this.data = data;
+        	this.next = null;
         } 
     } 
 
 
     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;
+        System.out.println(data + " pushed to 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 
+        if (root == null) {
+            System.out.println("Stack Underflow");
+            return 0;
+        } else {
+            int popped = root.data;
+            root = root.next;
+            return popped;
+        }
     } 
 
     public int peek() 
     { 
         //Write code to just return the topmost element without removing it.
+        if (root == null) {
+            System.out.println("Stack Underflow");
+            return 0;
+        } else {
+            return root.data;
+        }
     } 
 
 	//Driver code

Exercise_3.java

@@ -1,8 +1,7 @@
-import java.io.*; 
-
+
 // Java program to implement 
 // a Singly Linked List 
-public class LinkedList { 
+class LinkedList { 
 
     Node head; // head of list 
 
@@ -17,24 +16,35 @@ 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 
-
+        Node newNode = new Node(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 
+            if (list.head == null) {
+            list.head = newNode;
+        } else {
+            Node last = list.head;
+            while (last.next != null) {
+                last = last.next;
+            }
+            last.next = newNode;
+        }
         // Return the list by head 
-
+        return list;
     } 
 
     // Method to print the LinkedList. 
@@ -45,6 +55,11 @@ public static void printList(LinkedList list)
             // Print the data at current node 
 
             // Go to next node 
+        Node current = list.head;
+        while (current != null) {
+            System.out.println(current.data);
+            current = current.next;
+        }
     } 
 
     // Driver code