Completed Pre course 1

Exercise_1.java

@@ -1,46 +1,53 @@
-class Stack { 
-    //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 
-    } 
+class Stack {
+	// Time Complexity : O(1)
+    // Space Complexity : O(n) (n= MAX)
+    // Did this code successfully run on Leetcode : Yes (after updating the Class name)
+    // Any problem you faced while coding this : No
 
-    Stack() 
-    { 
-        //Initialize your constructor 
-    } 
-
-    boolean push(int x) 
-    { 
-        //Check for stack Overflow
-        //Write your code here
-    } 
-
-    int pop() 
-    { 
-        //If empty return 0 and print " Stack Underflow"
-        //Write your code here
-    } 
-
-    int peek() 
-    { 
-        //Write your code here
-    } 
-} 
-
-// Driver code 
-class Main { 
-    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"); 
-    } 
+	static final int MAX = 1000;
+	int top;
+	int a[] = new int[MAX]; // Maximum size of Stack
+
+	boolean isEmpty() {
+		return top < 0;
+	}
+
+	Stack() {
+		top = -1;
+		a = new int[MAX];
+	}
+
+	boolean push(int x) {
+		if (top < MAX - 1) {
+            a[++top] = x;
+            return true;
+        }
+        System.out.println("Stack Overflow");
+        return false;
+	}
+
+	int pop() {
+		if (top == -1) {
+			System.out.println("Stack Underflow");
+			return 0;
+		}
+		return a[top--];
+	}
+
+	int peek() {
+		if (isEmpty()) {
+			return 0;
+		}
+		return a[top];
+	}
+}
+
+class Main {
+	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");
+	}
 }

Exercise_2.java

@@ -1,52 +1,63 @@
-public class StackAsLinkedList { 
-
-    StackNode root; 
-
-    static class StackNode { 
-        int data; 
-        StackNode next; 
-
-        StackNode(int data) 
-        { 
-            //Constructor here 
-        } 
-    } 
-
-
-    public boolean isEmpty() 
-    { 
-        //Write your code here for the condition if stack is empty. 
-    } 
-
-    public void push(int data) 
-    { 
-        //Write code to push data to the 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 
-    } 
-
-    public int peek() 
-    { 
-        //Write code to just return the topmost element without removing it.
-    } 
-
-	//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()); 
-    } 
-} 
+public class StackAsLinkedList {
+
+	// Time Complexity : O(1)
+	// Space Complexity : O(n)
+	// Did this code successfully run on Leetcode : Yes
+	// Any problem you faced while coding this : No
+
+	StackNode root;
+
+	static class StackNode {
+		int data;
+		StackNode next;
+
+		StackNode(int data) {
+			this.data = data;
+		}
+	}
+
+	public boolean isEmpty() {
+		return root == null;
+	}
+
+	public void push(int data) {
+		if (isEmpty()) {
+			root = new StackNode(data);
+			return;
+		}
+		StackNode node = new StackNode(data);
+		node.next = root;
+		root = node;
+	}
+
+	public int pop() {
+		if (root == null) {
+			System.out.println("Stack Underflow");
+			return 0;
+		}
+		StackNode node = root;
+		root = root.next;
+		return node.data;
+	}
+
+	public int peek() {
+		if (root != null) {
+			return root.data;
+		}
+		return 0;
+	}
+
+	// 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());
+	}
+}

Exercise_3.java

@@ -1,70 +1,64 @@
-import java.io.*; 
-
-// Java program to implement 
-// a Singly Linked List 
-public class LinkedList { 
-
-    Node head; // head of list 
-
-    // Linked list Node. 
-    // This inner class is made static 
-    // so that main() can access it 
-    static class Node { 
-
-        int data; 
-        Node next; 
-
-        // Constructor 
-        Node(int d) 
-        { 
-            //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, 
-        // then make the new node as head 
-
-            // Else traverse till the last node 
-            // and insert the new_node there 
+import java.io.*;
 
-            // Insert the new_node at last node 
-        // Return the list by head 
-
-    } 
-
-    // Method to print the LinkedList. 
-    public static void printList(LinkedList list) 
-    {  
-        // Traverse through the LinkedList 
-
-            // Print the data at current node 
-
-            // Go to next node 
-    } 
-
-    // Driver code 
-    public static void main(String[] args) 
-    { 
-        /* Start with the empty list. */
-        LinkedList list = new LinkedList(); 
-
-        // 
-        // ******INSERTION****** 
-        // 
-
-        // Insert the values 
-        list = insert(list, 1); 
-        list = insert(list, 2); 
-        list = insert(list, 3); 
-        list = insert(list, 4); 
-        list = insert(list, 5); 
-
-        // Print the LinkedList 
-        printList(list); 
-    } 
+public class MyLinkedList {
+
+	// Time Complexity : O(n)
+	// Space Complexity : O(n)
+	// Did this code successfully run on Leetcode : Yes
+	// Any problem you faced while coding this : No
+
+	Node head;
+
+	static class Node {
+
+		int data;
+		Node next;
+
+		// Constructor
+		Node(int d) {
+			this.data = d;
+		}
+	}
+
+	// Method to insert a new node
+	public static MyLinkedList insert(MyLinkedList list, int data) {
+		// Create a new node with given data
+		Node node = new Node(data);
+		if (list.head == null) {
+			list.head = node;
+		} else {
+			Node n = list.head;
+			while (n.next != null) {
+				n = n.next;
+			}
+			n.next = node;
+		}
+
+		return list;
+	}
+
+	// Method to print the LinkedList.
+	public static void printList(MyLinkedList list) {
+		Node n = list.head;
+		while (n != null) {
+			System.out.println(n.data);
+			n = n.next;
+		}
+	}
+
+	// Driver code
+	public static void main(String[] args) {
+		/* Start with the empty list. */
+		MyLinkedList list = new MyLinkedList();
+
+		// Insert the values
+		list = insert(list, 1);
+		list = insert(list, 2);
+		list = insert(list, 3);
+		list = insert(list, 4);
+		list = insert(list, 5);
+
+		// Print the LinkedList
+		printList(list);
+	}
 }