From 4d1ef04414966763b6918fa5dbad7ee01befcdf5 Mon Sep 17 00:00:00 2001
From: ritvij14 <ritvij2001@gmail.com>
Date: Tue, 9 Mar 2021 09:52:21 +0530
Subject: [PATCH 1/2] added implementation of stack using Queue in Java

---
 Java/README.md               |  1 +
 Java/ds/StackUsingQueue.java | 96 ++++++++++++++++++++++++++++++++++++
 2 files changed, 97 insertions(+)
 create mode 100644 Java/ds/StackUsingQueue.java

diff --git a/Java/README.md b/Java/README.md
index 0b4d532484..035f37043f 100644
--- a/Java/README.md
+++ b/Java/README.md
@@ -104,6 +104,7 @@ _add list here_
 - [Sparse Matrix](ds/SparseMatrix.java)
 - [Stack ~ Linked List Implementation](ds/Stack.java)
 - [Stack](ds/Stackll.java) Implementation-of-queue-using-stack
+- [Implementation of Stack using Queue](ds/StackUsingQueue.java)
 - [Tree Traversal(preorder traversal)](ds/preOrderTraversal.java)
 - [Tree Traversal(postorder traversal)](ds/postorder_Traversal.java)
 - [Priority Queue](ds/PriorityQueueEg.java)
diff --git a/Java/ds/StackUsingQueue.java b/Java/ds/StackUsingQueue.java
new file mode 100644
index 0000000000..c5bd17fef4
--- /dev/null
+++ b/Java/ds/StackUsingQueue.java
@@ -0,0 +1,96 @@
+/* Java Program to implement a stack using two queues
+by making the push operation costly*/
+
+import java.util.*;
+
+public class StackUsingQueue {
+    static class Stack {
+        // the 2 queues
+        Queue<Integer> qu1 = new LinkedList<>();
+        Queue<Integer> qu2 = new LinkedList<>();
+
+        // storing current size
+        int currentSize;
+
+        Stack() {
+            currentSize = 0;
+        }
+
+        void push(int num) {
+            currentSize++;
+
+            // first, push num into an empty qu2
+            qu2.add(num);
+
+            // now we push the other elements from qu1 to qu2
+            while (!qu1.isEmpty()) {
+                qu2.add(qu1.peek());
+                qu1.remove();
+            }
+        }
+
+        void pop() {
+            if (qu2.isEmpty()) {
+                return;
+            }
+            qu2.remove();
+            currentSize--;
+        }
+
+        int top() {
+            if (qu2.isEmpty()) {
+                return -1;
+            }
+            return qu2.peek();
+        }
+
+        int size() {
+            return currentSize;
+        }
+    }
+
+    public static void main(String[] args) {
+        Stack stack = new Stack();
+
+        // taking input
+        Scanner sc = new Scanner(System.in);
+        System.out.println("Enter size:");
+        int size = sc.nextInt();
+        System.out.println("Enter the numbers:");
+        for (int i = 0; i < size; i++) {
+            stack.push(sc.nextInt());
+        }
+        sc.close();
+
+        // giving output
+        System.out.println("Current Size:" + stack.size());
+        for (int i = size; i > 0; i--) {
+            System.out.println(stack.top());
+            stack.pop();
+        }
+        System.out.println("Current Size:" + stack.size());
+    }
+}
+
+/**
+ * Sample input/output:
+ * 
+ * Enter size:
+ * 5
+ * Enter the numbers:
+ * 5
+ * 4
+ * 3
+ * 2
+ * 1
+ * Current Size:5
+ * 5
+ * 4
+ * 3
+ * 2
+ * 1
+ * Current Size:0
+ * 
+ * Time complexity: O(n) for push, O(1) for pop
+ * Space complexity: O(n)
+ */
\ No newline at end of file

From f3b4f24fa6cc2fe9b029ed5374941a6c357fd063 Mon Sep 17 00:00:00 2001
From: ritvij14 <ritvij2001@gmail.com>
Date: Tue, 9 Mar 2021 11:17:37 +0530
Subject: [PATCH 2/2] minor modifications

---
 Java/ds/StackUsingQueue.java | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/Java/ds/StackUsingQueue.java b/Java/ds/StackUsingQueue.java
index c5bd17fef4..dc5ec2fdf3 100644
--- a/Java/ds/StackUsingQueue.java
+++ b/Java/ds/StackUsingQueue.java
@@ -33,6 +33,8 @@ void pop() {
             if (qu2.isEmpty()) {
                 return;
             }
+
+            // remove element and reduce size if qu2 not empty
             qu2.remove();
             currentSize--;
         }
@@ -41,6 +43,8 @@ int top() {
             if (qu2.isEmpty()) {
                 return -1;
             }
+
+            // return the top element
             return qu2.peek();
         }
 
@@ -52,7 +56,6 @@ int size() {
     public static void main(String[] args) {
         Stack stack = new Stack();
 
-        // taking input
         Scanner sc = new Scanner(System.in);
         System.out.println("Enter size:");
         int size = sc.nextInt();
@@ -62,7 +65,6 @@ public static void main(String[] args) {
         }
         sc.close();
 
-        // giving output
         System.out.println("Current Size:" + stack.size());
         for (int i = size; i > 0; i--) {
             System.out.println(stack.top());
@@ -93,4 +95,4 @@ public static void main(String[] args) {
  * 
  * Time complexity: O(n) for push, O(1) for pop
  * Space complexity: O(n)
- */
\ No newline at end of file
+ */