232. 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 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).

这道题可以借助另一个stack进行前后转移，左右颠倒，代码如下：

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