completed precourse-1

Exercise_1.java

@@ -1,3 +1,11 @@
+// Time Complexity : O(1)
+// Space Complexity : O(n)
+// Did this code successfully run on Leetcode : yes - implement stack using queues
+// Any problem you faced while coding this :
+
+
+// Your code here along with comments explaining your approach
+
 class Stack { 
     //Please read sample.java file before starting.
   //Kindly include Time and Space complexity at top of each file
@@ -8,28 +16,53 @@ class Stack {
     boolean isEmpty() 
     { 
         //Write your code here 
+        if(a.length == 0)
+        {
+            return true;
+        }
+        return false;
     } 
 
     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;
+        }
+        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 0;
+        }
+        return a[top--];
     } 
 
     int peek() 
     { 
         //Write your code here
+        if(isEmpty())
+        {
+            System.out.println("Stack is empty");
+            return -1;
+        }
+        return a[top];
     } 
 } 
 

Exercise_3.java → LinkedList.java

@@ -1,10 +1,25 @@
+// Time Complexity :O(1) for insert and O(n) for iterate
+// Space Complexity : O(n)
+// Did this code successfully run on Leetcode :
+// Any problem you faced while coding this :
+
+
+// Your code here along with comments explaining your approach
+
 import java.io.*; 
 
 // Java program to implement 
 // a Singly Linked List 
 public class LinkedList { 
 
     Node head; // head of list 
+    int size;
+
+    LinkedList()
+    {
+        this.head = null;
+        this.size = 0;
+    }
 
     // Linked list Node. 
     // This inner class is made static 
@@ -18,13 +33,16 @@ static class Node {
         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 
@@ -34,7 +52,21 @@ public static LinkedList insert(LinkedList list, int data)
 
             // Insert the new_node at last node 
         // Return the list by head 
-
+
+        if(list.size == 0)
+        {
+            list.head = newNode;
+            list.size++;
+            return list;
+        }
+        Node curr = list.head;
+        while(curr.next != null)
+        {
+            curr = curr.next;
+        }
+        curr.next = newNode;
+        list.size++;
+        return list;
     } 
 
     // Method to print the LinkedList. 
@@ -44,7 +76,18 @@ public static void printList(LinkedList list)
 
             // Print the data at current node 
 
-            // Go to next node 
+            // Go to next node
+        if(list.size == 0)
+        {
+            System.out.println("List is empty");
+            return;
+        } 
+        Node curr = list.head;
+        while(curr != null)
+        {
+            System.out.println(curr.data);
+            curr = curr.next;
+        }
     } 
 
     // Driver code 

StackAsLinkedList.java

@@ -0,0 +1,102 @@
+// Time Complexity : O(1) for push, isEmpty, and peek and O(n) for pop
+// Space Complexity : O(n)
+// Did this code successfully run on Leetcode :yes - implement stack using queues
+// Any problem you faced while coding this :
+
+
+// Your code here along with comments explaining your approach
+public class StackAsLinkedList { 
+
+    StackNode root; 
+    StackNode top;
+
+    StackAsLinkedList()
+    {
+        this.root = null;
+        this.top = null;
+    }
+
+    static class StackNode { 
+        int data; 
+        StackNode next; 
+
+        StackNode(int data) 
+        { 
+            //Constructor here 
+            this.data = data;
+            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);
+        if(null == root)
+        {
+            root = newNode;
+            top = newNode;
+        }
+        else
+        {
+            top.next = newNode;
+            top = 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 topElement = top.data;
+        StackNode curr = root;
+        while(curr.next != top)
+        {
+            curr = curr.next;
+        }
+        top = curr;
+        top.next = null;
+
+        return topElement;
+    } 
+
+    public int peek() 
+    { 
+        //Write code to just return the topmost element without removing it.
+        if(isEmpty())
+        {
+            System.out.println("Stack is empty");
+            return -1;
+        }
+        return top.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()); 
+    } 
+}