Precourse-1 Done

Exercise_1.java

@@ -1,36 +1,56 @@
-class Stack { 
+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 
-    } 
+    //Kindly include Time and Space complexity at top of each file
+    // Time Complexity :
+    // Push (O(1)), Pop(O(1)), Peek(O(1))
+    // Space Complexity :
+    // O(1), No additional space used, just the array to keep the values
+    // Did this code successfully run on Leetcode :
+    // Didn't find exact problem on leetcode
+    // Any problem you faced while coding this :
+    // No
+    // Approach :
+    // Insert value to/remove value from the end of the array using top as pointer.
 
-    Stack() 
-    { 
-        //Initialize your constructor 
-    } 
-
-    boolean push(int x) 
-    { 
+    static final int MAX = 1000;
+    int top;
+    int[] a = new int[MAX]; // Maximum size of Stack
+
+    boolean isEmpty() {
+        return top == 0;
+    }
+
+    Stack() {
+        //Initialize your constructor
+    }
+
+    boolean push(int x) {
         //Check for stack Overflow
         //Write your code here
-    } 
-
-    int pop() 
-    { 
+        if (top >= MAX) {
+            return false;
+        }
+        a[top++] = x;
+        return true;
+    }
+
+    int pop() {
         //If empty return 0 and print " Stack Underflow"
         //Write your code here
-    } 
-
-    int peek() 
-    { 
+        if (top == 0) {
+            System.out.println("Stack Underflow");
+            return 0;
+        }
+        int topMost = a[top - 1];
+        top--;
+        return topMost;
+    }
+
+    int peek() {
         //Write your code here
-    } 
+        if (top == 0 || top == MAX) return 0;
+        return a[top - 1];
+    }
 } 
 
 // Driver code 
@@ -41,6 +61,6 @@ public static void main(String args[])
         s.push(10); 
         s.push(20); 
         s.push(30); 
-        System.out.println(s.pop() + " Popped from stack"); 
-    } 
+        System.out.println(s.pop() + " Popped from stack");
+    }
 }

Exercise_2.java

@@ -1,52 +1,80 @@
-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()); 
-    } 
-} 
+class StackAsLinkedList {
+
+    // Time Complexity :
+    // Push (O(1)), Pop (O(1))
+    // Space Complexity :
+    // Push (O(1)), Pop (O(1)), No additional space needed
+    // Did this code successfully run on Leetcode :
+    // Yes
+    // Any problem you faced while coding this :
+    // Didn't find exact problem on leetcode
+    // Approach :
+    // Keep reference of root node as the top of the stack. Create new node and make it the new top/root node.
+    StackNode root;
+
+    static class StackNode {
+        int data;
+        StackNode next;
+
+        StackNode(int data)
+        {
+            this.data = data;
+            this.next = null;
+        }
+    }
+
+
+    public boolean isEmpty()
+    {
+        return root == null;
+    }
+
+    public void push(int data)
+    {
+        StackNode newNode = new StackNode(data);
+        if(root == null){
+            root = newNode;
+            return ;
+        }
+        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
+        if(root == null){
+            System.out.println("Stack Underflow");
+            return 0;
+        }
+        StackNode top = root;
+        root = root.next;
+        return top.data;
+    }
+
+    public int peek()
+    {
+        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(); 
+
+        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

@@ -1,50 +1,66 @@
 import java.io.*; 
 
 // Java program to implement 
-// a Singly Linked List 
-public class LinkedList { 
-
+// a Singly Linked List
+
+// Time Complexity :
+// Insert - O(1)
+// Space Complexity :
+// O(1)
+// Did this code successfully run on Leetcode :
+// Didn't find exact problem on leetcode
+// Any problem you faced while coding this :
+// No
+class LinkedList {
+
     Node head; // head of list 
-
+    Node tail;
     // Linked list Node. 
     // This inner class is made static 
     // so that main() can access it 
-    static class Node { 
-  
-        int data; 
-        Node next; 
-  
+    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 
+        Node(int d)
+        {
+            this.data = d;
+            this.next = null;
+        }
+    }
 
-            // 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 
+    // Method to insert a new node
+    public static LinkedList insert(LinkedList list, int data)
+    {
+        // Create a new node with given data
+        Node pointer = list.head;
+        Node newNode = new Node(data);
+        // If the Linked List is empty,
+        // then make the new node as head
+        if(pointer == null){
+            list.head = newNode;
+            list.tail = newNode;
+            return list;
+        }
+        // Else insert the new node at the tail end of linked list
+        pointer = list.tail;
+        pointer.next = newNode;
+        list.tail = newNode;
+        return list;
+    }
+
+    // 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
+        Node pointer = list.head;
+        while(pointer != null){
+            System.out.print(pointer.data+"\t");
+            pointer = pointer.next;
+        }
     } 
 
     // Driver code 
@@ -63,7 +79,7 @@ public static void main(String[] args)
         list = insert(list, 3); 
         list = insert(list, 4); 
         list = insert(list, 5); 
-  
+
         // Print the LinkedList 
         printList(list); 
     }