Implement_Stack_Using_Queues.cpp

PROBLEM STATEMENT :-
  Implement a last-in-first-out (LIFO) stack using only two queues. The implemented stack should support all the functions of a normal stack (push, top, pop, and empty).

Implement the MyStack class:

void push(int x) Pushes element x to the top of the stack.
int pop() Removes the element on the top of the stack and returns it.
int top() Returns the element on the top of the stack.
boolean empty() Returns true if the stack is empty, false otherwise.
Notes:

You must use only standard operations of a queue, which means that only push to back, peek/pop from front, size and is empty operations are valid.
Depending on your language, the queue may not be supported natively. You may simulate a queue using a list or deque (double-ended queue) as long as you use only a queue's standard operations.
 

Example 1:

Input
["MyStack", "push", "push", "top", "pop", "empty"]
[[], [1], [2], [], [], []]
Output
[null, null, null, 2, 2, false]

Explanation
MyStack myStack = new MyStack();
myStack.push(1);
myStack.push(2);
myStack.top(); // return 2
myStack.pop(); // return 2
myStack.empty(); // return False


SOLUTION :- 
  
  class MyStack {
public:
    /** Initialize your data structure here. */
    queue<int> q1;
    queue<int> q2;
    
    MyStack() {
        
    }
    
    /** Push element x onto stack. */
    void push(int x) {
        while(q1.empty()!=true)
        {
            q2.push(q1.front());
            q1.pop();
        }
        q1.push(x);
        while(!q2.empty())
        {
            q1.push(q2.front());
            q2.pop();
        }
    }
    
    /** Removes the element on top of the stack and returns that element. */
    int pop() 
    {
        while(!q1.empty())
        {
            q2.push(q1.front());
            q1.pop();
        }
        int r=q2.front();
        q2.pop();
         while(!q2.empty())
        {
            q1.push(q2.front());
            q2.pop();
        }
        return r;
        
    }
    
    /** Get the top element. */
    int top()
    {
        while(!q1.empty())
        {
            q2.push(q1.front());
            q1.pop();
        }
        
        int x=q2.front();
        
         while(!q2.empty())
        {
            q1.push(q2.front());
            q2.pop();
        }
        return x;
    }
    
    /** Returns whether the stack is empty. */
    bool empty() 
    {
        
        if(q1.size()==0)
            return true;
        else
            return false;
        
    }
};

/**
 * Your MyStack object will be instantiated and called as such:
 * MyStack* obj = new MyStack();
 * obj->push(x);
 * int param_2 = obj->pop();
 * int param_3 = obj->top();
 * bool param_4 = obj->empty();
 */