Merge pull request matthewsamuel95#283 from CircleJerkHug/master

Implementation of Stack using Queue

Author: matthewsamuel95

.gitignore

@@ -0,0 +1,3 @@
+.pyc
+a.out
+*.exe

Data Structures/Queue/queue_from_stack.cpp

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+// CPP program to implement Queue using 
+// one stack and recursive call stack. 
+#include <bits/stdc++.h> 
+using namespace std; 
+  
+struct Queue { 
+    stack<int> s; 
+  
+    // Enqueue an item to the queue 
+    void enQueue(int x) 
+    { 
+        s.push(x); 
+    } 
+  
+    // Dequeue an item from the queue 
+    int deQueue() 
+    { 
+        if (s.empty()) { 
+            cout << "Q is empty"; 
+            exit(0); 
+        } 
+  
+        // pop an item from the stack 
+        int x = s.top(); 
+        s.pop(); 
+  
+        // if stack becomes empty, return 
+        // the popped item 
+        if (s.empty()) 
+            return x; 
+  
+        // recursive call 
+        int item = deQueue(); 
+  
+        // push popped item back to the stack 
+        s.push(x); 
+  
+        // return the result of deQueue() call 
+        return item; 
+    } 
+}; 
+  
+// Driver code 
+int main() 
+{ 
+    Queue q; 
+    q.enQueue(1); 
+    q.enQueue(2); 
+    q.enQueue(3); 
+  
+    cout << q.deQueue() << '\n'; 
+    cout << q.deQueue() << '\n'; 
+    cout << q.deQueue() << '\n'; 
+  
+    return 0; 
+} 

Data Structures/Stack/stack_from_queue.cpp

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+// Program to implement a stack using two queue 
+#include<bits/stdc++.h> 
+using namespace std; 
+  
+class Stack 
+{  
+    // Two inbuilt queues 
+    queue<int> q1, q2; 
+      
+    // To maintain current number of 
+    // elements 
+    int curr_size; 
+  
+    public: 
+    Stack() 
+    { 
+        curr_size = 0; 
+    } 
+  
+    void push(int x) 
+    { 
+        curr_size++; 
+  
+        // Push x first in empty q2 
+        q2.push(x); 
+  
+        // Push all the remaining  
+        // elements in q1 to q2.  
+        while (!q1.empty()) 
+        { 
+            q2.push(q1.front()); 
+            q1.pop(); 
+        } 
+  
+        // swap the names of two queues 
+        queue<int> q = q1; 
+        q1 = q2; 
+        q2 = q; 
+    } 
+  
+    void pop(){ 
+  
+        // if no elements are there in q1  
+        if (q1.empty()) 
+            return ; 
+        q1.pop(); 
+        curr_size--; 
+    } 
+  
+    int top() 
+    { 
+        if (q1.empty()) 
+            return -1; 
+        return q1.front(); 
+    } 
+  
+    int size() 
+    { 
+        return curr_size; 
+    } 
+}; 
+  
+// driver code 
+int main() 
+{ 
+    Stack s; 
+    s.push(1); 
+    s.push(2); 
+    s.push(3); 
+  
+    cout << "current size: " << s.size()  
+         << endl; 
+    cout << s.top() << endl; 
+    s.pop(); 
+    cout << s.top() << endl; 
+    s.pop(); 
+    cout << s.top() << endl; 
+  
+    cout << "current size: " << s.size()  
+         << endl; 
+    return 0; 
+}