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()