Completed PreCourse-1

Exercise_1.java

@@ -1,35 +1,57 @@
+
+//Please read sample.java file before starting.
+//Kindly include Time and Space complexity at top of each file
+
+// Time Complexity :O(1)
+// Space Complexity :O(MAX) (MAX is size of stack)
+// Did this code successfully run on Leetcode : ran code locally
+// Any problem you faced while coding this :no
+
+
+// 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
     static final int MAX = 1000; 
     int top; 
     int a[] = new int[MAX]; // Maximum size of Stack 
 
+    //check if array is empty by checking top value
     boolean isEmpty() 
     { 
-        //Write your code here 
+        return top==-1;
     } 
 
     Stack() 
     { 
-        //Initialize your constructor 
+        top = -1;
     } 
 
     boolean push(int x) 
     { 
-        //Check for stack Overflow
-        //Write your code here
+        //top shouldnot be >MAX
+        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
-    } 
+    int pop() { 
+        //if top is -1 then array is empty
+        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 0; 
+        } 
+        return a[top];
     } 
 } 
 

Exercise_2.java

@@ -1,3 +1,12 @@
+// Time Complexity :O(1)
+// Space Complexity :O(n)
+// Did this code successfully run on Leetcode :ran code locally
+// Any problem you faced while coding this :no
+
+
+// Your code here along with comments explaining your approach
+
+
 public class StackAsLinkedList { 
 
     StackNode root; 
@@ -8,31 +17,44 @@ static class StackNode {
 
         StackNode(int data) 
         { 
-            //Constructor here 
+            this.data = data;
+            this.next = null;
         } 
     } 
 
-
+	//check if stack empty
     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;   //point node to top
+        root = newnode;   //update top to 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 popped = root.data; 
+        root = root.next;   //move top to next node
+        return popped;
     } 
 
     public int peek() 
     { 
-        //Write code to just return the topmost element without removing it.
+        if (isEmpty()) { 
+            System.out.println("Stack is Empty"); 
+            return 0; 
+        } 
+        return root.data;
     } 
 
 	//Driver code

Exercise_3.java

@@ -1,6 +1,14 @@
 import java.io.*; 
 
 // Java program to implement 
+// Time Complexity :O(n) for insert and printList
+// Space Complexity :O(n)
+// Did this code successfully run on Leetcode :ran code locally
+// Any problem you faced while coding this :no
+
+
+// Your code here along with comments explaining your approach
+
 // a Singly Linked List 
 public class LinkedList { 
 
@@ -17,34 +25,48 @@ static class Node {
         // Constructor 
         Node(int d) 
         { 
-            //Write your code here 
+            data = d;
+            next = null;
         } 
     } 
 
     // 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 
-
-            // Insert the new_node at last node 
-        // Return the list by head 
-
+        Node newnode = new Node(data);
+        // if linkedlist is empty then make newnode as head
+        if (list.head == null) { 
+            list.head = newnode; 
+        } 
+        // else traverse till the last node and insert there 
+        else { 
+            Node currnode = list.head; 
+
+            // traverse till the last node 
+            while (currnode.next != null) { 
+                currnode = currnode.next; 
+            } 
+
+            // Insert the newnode at last node 
+            currnode.next = 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 currnode = list.head; 
+
+        // traverse the LinkedList 
+        while (currnode != null) { 
+            System.out.print(currnode.data + " "); 
+
+            // go to next node 
+            currnode = currnode.next; 
+        } 
+        System.out.println(); 
     } 
 
     // Driver code