Add Reverse Stack (#2653)

DataStructures/Stacks/ReverseStack.java

@@ -0,0 +1,70 @@
+package DataStructures.Stacks;
+
+import java.util.Scanner;
+import java.util.Stack;
+
+/**
+ * Reversal of a stack using recursion. 
+ *
+ * @author Ishika Agarwal, 2021
+ */
+
+public class ReverseStack {
+
+    public static void main(String args[]){
+
+        Scanner sc = new Scanner(System.in);
+        System.out.println("Enter the number of elements you wish to insert in the stack");
+        int n = sc.nextInt();
+        int i;
+        Stack<Integer> stack = new Stack<Integer>();
+        System.out.println("Enter the stack elements");
+        for(i = 0; i < n ; i++)
+            stack.push(sc.nextInt());
+        sc.close();
+        reverseStack(stack);
+        System.out.println("The reversed stack is:");
+        while(!stack.isEmpty()){
+            System.out.print(stack.peek()+",");
+            stack.pop();
+        }
+
+    }
+
+    private static void reverseStack(Stack<Integer> stack) {
+        if(stack.isEmpty()){
+            return;
+        }
+
+        //Store the topmost element
+        int element = stack.peek();
+        //Remove the topmost element
+        stack.pop();
+
+        //Reverse the stack for the leftover elements
+        reverseStack(stack);
+
+        //Insert the topmost element to the bottom of the stack
+        insertAtBottom(stack,element);
+
+    }
+
+    private static void insertAtBottom(Stack<Integer> stack, int element) {
+        
+        if(stack.isEmpty()){
+            //When stack is empty, insert the element so it will be present at the bottom of the stack
+            stack.push(element);
+            return;
+        }
+
+        int ele = stack.peek();
+        /*Keep popping elements till stack becomes empty. Push the elements once the topmost element has
+            moved to the bottom of the stack.
+        */
+        stack.pop();
+        insertAtBottom(stack, element);
+
+        stack.push(ele);
+    }
+    
+}