[leedcode 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 MyQueue {
      //用两个栈实现队列
      //push时push到一个栈，pop时，需要先判断2栈是否为空，如果不为空，直接弹2栈的栈顶，如果为空，需要将1栈的元素转移到2栈中
      //注意stack判空empty和isEmpty都可以使用
      Stack<Integer> s1=new Stack<Integer>();
      Stack<Integer> s2=new Stack<Integer>();
      // Push element x to the back of queue.
      public void push(int x) {
         s1.push(x); 
      }
  
      // Removes the element from in front of queue.
      public void pop() {
         if(!s2.isEmpty()){
             s2.pop();
         }else{
             while(!s1.isEmpty()){
                s2.push(s1.pop()); 
             }
             s2.pop();
         }
      }
  
      // Get the front element.
      public int peek() {
          if(!s2.empty()){
             return s2.peek();
         }else{
             while(!s1.empty()){
                s2.push(s1.pop()); 
             }
             return s2.peek();
         }
      }
  
      // Return whether the queue is empty.
      public boolean empty() {
          return s1.isEmpty()&&s2.empty();
      }
  }