225. Implement Stack using Queues

Implement the following operations of a stack using queues.

• push(x) -- Push element x onto stack.
• pop() -- Removes the element on top of the stack.
• top() -- Get the top element.
• empty() -- Return whether the stack is empty.

Notes:

• You must use only standard operations of a queue -- which means only push to back, peek/pop from front, size, and is empty operations are valid.
• Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.
• You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).

  class Stack {
  public:
      queue<int> m1;
      queue<int> m2;
      // Push element x onto stack.
      void push(int x) 
      {
          if(m1.empty())
          {
              m1.push(x);
              while(!m2.empty())
              {
                  int temp=m2.front();
                  m2.pop();
                  m1.push(temp);
              }
          }
          else
          {
              m2.push(x);
              while(!m1.empty())
              {
                  int temp=m1.front();
                  m1.pop();
                  m2.push(temp);
              }
          }
      }
  
      // Removes the element on top of the stack.
      void pop() 
      {
          if(!m1.empty())
              m1.pop();
          if(!m2.empty())
              m2.pop();        
      }
  
      // Get the top element.
      int top() 
      {
          if(!m1.empty())
          {
              int y=m1.front();
              return y;
          }
          if(!m2.empty())
          {
              int y=m2.front();
              return y;
          }
      }
  
      // Return whether the stack is empty.
      bool empty() 
      {
          return m1.empty()&&m2.empty();
      }
  };