Merge pull request HarshCasper#2311 from ritvij14/stack-queue

added implementation of stack using Queue in Java

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)

Java/ds/StackUsingQueue.java

@@ -0,0 +1,98 @@
+/* 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;
+            }
+
+            // remove element and reduce size if qu2 not empty
+            qu2.remove();
+            currentSize--;
+        }
+
+        int top() {
+            if (qu2.isEmpty()) {
+                return -1;
+            }
+
+            // return the top element
+            return qu2.peek();
+        }
+
+        int size() {
+            return currentSize;
+        }
+    }
+
+    public static void main(String[] args) {
+        Stack stack = new Stack();
+
+        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();
+
+        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)
+ */