LeetCode 232. Implement Queue using Stacks 用栈实现队列（Java）

题目：

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.

Example:
MyQueue queue = new MyQueue();
queue.push(1);
queue.push(2);
queue.peek(); // returns 1
queue.pop(); // returns 1
queue.empty(); // returns false

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).

解答：

队列的特点是先进先出，栈的特点是先进后出
思路：通过两个栈进行操作，一个栈 stack1 只作为压入栈，在压入数据时只往这个栈中压入。另一个栈 stack2 只作为弹出栈，在弹出数据时只从这个栈弹出。这样出栈时就通过栈的逆序放入另一个栈中。

class MyQueue {
    Stack<Integer> stack1;
    Stack<Integer> stack2;
    
    /** Initialize your data structure here. */
    public MyQueue() {
        stack1 = new Stack<Integer>();
        stack2 = new Stack<Integer>();
    }
    
    /** Push element x to the back of queue. */
    public void push(int x) {
        while(!stack2.isEmpty()) {
            stack1.push(stack2.pop());
        }
        stack1.push(x);
    }
    
    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        while(!stack1.isEmpty()) {
            stack2.push(stack1.pop());
        }
        return stack2.pop();
    }
    
    /** Get the front element. */
    public int peek() {
        while(!stack1.isEmpty()) {
            stack2.push(stack1.pop());
        }
        return stack2.peek();
    }
    
    /** Returns whether the queue is empty. */
    public boolean empty() {
        return stack1.isEmpty() && stack2.isEmpty();
    }
}