题目描述

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).

分析

参考：LeetCode 225 Implement Stack using Queues

非常经典的题目，定义两个栈模拟队列。

1. push向stack1
2. pop从stack2，如果stack2为空，先将stack1的元素放入stack2

代码

    class MyQueue {

        Stack<Integer> stack1 = new Stack<Integer>();
        Stack<Integer> stack2 = new Stack<Integer>();

        // Push element x to the back of queue.
        public void push(int x) {
            stack1.push(x);
        }

        // Removes the element from in front of queue.
        public void pop() {
            if (stack2.isEmpty()) {
                if (stack1.isEmpty()) {
                    throw new IllegalStateException();
                }
                while (!stack1.isEmpty()) {
                    stack2.push(stack1.pop());
                }
            }
            stack2.pop();
        }

        // Get the front element.
        public int peek() {
            if (stack2.isEmpty()) {
                if (stack1.isEmpty()) {
                    throw new IllegalStateException();
                }
                while (!stack1.isEmpty()) {
                    stack2.push(stack1.pop());
                }
            }
            return stack2.peek();
        }

        // Return whether the queue is empty.
        public boolean empty() {

            if (stack1.isEmpty() && stack2.isEmpty()) {
                return true;
            }

            return false;
        }
    }