232 Implement Queue using Stacks

Implement the following operations of a queue using stacks.

• push(x) -- Push element x to the back of queue.
• pop() -- Removes the element from in front of queue.
• peek() -- Get the front element.
• empty() -- Return whether the queue is empty.

Notes:

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

   

  class Queue {
      stack<int> inStk,outStk;
  public:
      // Push element x to the back of queue.
      void push(int x) {
          inStk.push(x);
      }
  
      // Removes the element from in front of queue.
      void pop(void) {
          if(outStk.empty())
          {
              while(!inStk.empty())
              {
                  outStk.push(inStk.top());
                  inStk.pop();
              }
          }
          
          outStk.pop();
      }
  
      // Get the front element.
      int peek(void) {
          if(outStk.empty())
          {
              while(!inStk.empty())
              {
                  outStk.push(inStk.top());
                  inStk.pop();
              }
          }
          
          return outStk.top();        
      }
  
      // Return whether the queue is empty.
      bool empty(void) {
          return inStk.empty() && outStk.empty();
      }
  };