added cpp implementation for queue<->stack

Author: vishal-wadhwa

Data Structures/QueueFromStack_StackFromQueue/Cpp/queue_from_stack.cpp

@@ -0,0 +1,84 @@
+#include <stack>
+#include <cassert>
+
+template<class T>
+class queue_from_stack {
+  std::stack<T> st1, st2;
+	void transfer();
+public:
+	queue_from_stack();
+  bool empty() const;
+  size_t size() const;
+	T &front();
+  void push(const T &);
+  void pop();
+};
+
+template<class T>
+queue_from_stack<T>::queue_from_stack() {}
+
+template<class T>
+void queue_from_stack<T>::transfer() {
+	while(!st1.empty()) {
+		st2.push(st1.top());
+		st1.pop();
+	}
+}
+
+template<class T>
+bool queue_from_stack<T>::empty() const {
+  return st1.empty() && st2.empty();
+}
+
+template<class T>
+size_t queue_from_stack<T>::size() const {
+  return st1.size() + st2.size();
+}
+
+template<class T>
+T &queue_from_stack<T>::front() {
+  if (st2.empty()) transfer();
+  return st2.top();
+}
+
+template<class T>
+void queue_from_stack<T>::push(const T &x) {
+	st1.push(x);
+}
+
+// removes the element at front
+template<class T>
+void queue_from_stack<T>::pop() {
+	if (st2.empty()) transfer();
+	st2.pop();
+}
+
+int main() {
+
+	queue_from_stack<int> qfs;
+	assert(qfs.size() == 0);
+	assert(qfs.empty());
+
+	qfs.push(1);
+	qfs.push(2);
+	qfs.push(3);
+
+	assert(qfs.front() == 1);
+
+	qfs.pop();
+
+	assert(qfs.front() == 2);
+
+	qfs.pop();
+	qfs.push(4);
+	qfs.push(5);
+
+	assert(qfs.size() == 3);
+	assert(qfs.front() == 3);
+
+	qfs.pop();
+
+	assert(qfs.front() == 4);
+
+	return 0;
+}

Data Structures/QueueFromStack_StackFromQueue/Cpp/stack_from_queue.cpp

@@ -0,0 +1,78 @@
+#include <queue>
+#include <cassert>
+
+template<class T>
+class stack_from_queue {
+  std::queue<T> q1, q2;
+public:
+	stack_from_queue();
+  bool empty() const;
+  size_t size() const;
+	T &top();
+  void push(const T &);
+  void pop();
+};
+
+template<class T>
+stack_from_queue<T>::stack_from_queue() {}
+
+template<class T>
+bool stack_from_queue<T>::empty() const {
+  return q1.empty();
+}
+
+template<class T>
+size_t stack_from_queue<T>::size() const {
+  return q1.size();
+}
+
+template<class T>
+T &stack_from_queue<T>::top() {
+  return q1.front();
+}
+
+template<class T>
+void stack_from_queue<T>::push(const T &x) {
+	std::queue<T> qtemp;
+  qtemp.push(x);
+  while(!q1.empty()) {
+    qtemp.push(q1.front());
+    q1.pop();
+  }
+  std::swap(q1, qtemp);
+}
+
+template<class T>
+void stack_from_queue<T>::pop() {
+	q1.pop();
+}
+
+int main() {
+
+	stack_from_queue<int> sfq;
+	assert(sfq.size() == 0);
+	assert(sfq.empty());
+
+	sfq.push(1);
+	sfq.push(2);
+	sfq.push(3);
+
+	assert(sfq.top() == 3);
+
+	sfq.pop();
+
+	assert(sfq.top() == 2);
+
+	sfq.pop();
+	sfq.push(4);
+	sfq.push(5);
+
+	assert(sfq.size() == 3);
+	assert(sfq.top() == 5);
+
+	sfq.pop();
+
+	assert(sfq.top() == 4);
+
+	return 0;
+}