LeetCode 232 Implement Queue using Stacks

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;        }    }