Java [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).

解题思路：

使用两个stack，一个用于插入数据，另一个用于输出数据。

代码如下：

class MyQueue {
    Stack<Integer> input = new Stack<Integer>();
    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.
    public void pop() {
        peek();
        output.pop();
    }

    // Get the front element.
    public int peek() {
        if(output.empty()){
            while(!input.empty()){
                output.push(input.pop());
            }
        }
        return output.peek();
    }

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