Implement Queue using Stacks（用两个栈实现队列）

来源：https://leetcode.com/problems/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 emptyoperations 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).

栈A负责push，栈B负责pop和peek，当pop或peek时，如果栈B没有元素，就将栈A中的内容按栈的pop顺序push到栈B中

Java

 class MyQueue {
     Stack<Integer> input;
     Stack<Integer> output;
     /** Initialize your data structure here. */
     public MyQueue() {
         input = new Stack<Integer>();
         output = new Stack<Integer>();
     }

     /** Push element x to the back of queue. */
     public void push(int x) {
         input.push(x);
     }

     /** Removes the element from in front of queue and returns that element. */
     public int pop() {
         peek();
         return output.pop();
     }

     /** Get the front element. */
     public int peek() {
         if(output.empty()) {
             int len = input.size();
             for(int i=0; i<len; i++) {
                 output.push(input.pop());
             }
         }
         return output.peek();
     }

     /** Returns whether the queue is empty. */
     public boolean empty() {
         return input.empty() && output.empty();
     }
 }

 /**
  * Your MyQueue object will be instantiated and called as such:
  * MyQueue obj = new MyQueue();
  * obj.push(x);
  * int param_2 = obj.pop();
  * int param_3 = obj.peek();
  * boolean param_4 = obj.empty();
  */

Python

 class MyQueue(object):
     i_stack = []
     o_stack = []
     def __init__(self):
         """
         Initialize your data structure here.
         """
         self.i_stack = []
         self.o_stack = []

     def push(self, x):
         """
         Push element x to the back of queue.
         :type x: int
         :rtype: void
         """
         self.i_stack.append(x)

     def pop(self):
         """
         Removes the element from in front of queue and returns that element.
         :rtype: int
         """
         self.peek()
         return self.o_stack.pop()

     def peek(self):
         """
         Get the front element.
         :rtype: int
         """
         if len(self.o_stack) == 0:
             i_len = len(self.i_stack)
             for i in range(i_len):
                 self.o_stack.append(self.i_stack.pop())
         return self.o_stack[-1]

     def empty(self):
         """
         Returns whether the queue is empty.
         :rtype: bool
         """
         return len(self.i_stack) == 0 and len(self.o_stack) == 0

 # Your MyQueue object will be instantiated and called as such:
 # obj = MyQueue()
 # obj.push(x)
 # param_2 = obj.pop()
 # param_3 = obj.peek()
 # param_4 = obj.empty()