232. Implement Queue using Stacks （用栈实现队列）-蒲公英云

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

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).

class MyQueue {

private LinkedList<Integer> stack1 = new LinkedList<Integer>(),
        stack2 = new LinkedList<Integer>();
// Push element x to the back of queue.
public void push(int x) {
    stack1.addFirst(x);
}
// Removes the element from in front of queue.
public void pop() {
    if (stack2.isEmpty()) {
        for (int i : stack1)
            stack2.addFirst(i);
        stack1.clear();
    }
    stack2.removeFirst();
}
// Get the front element.
public int peek() {
    if (stack2.isEmpty()) {
        for (int i : stack1)
            stack2.addFirst(i);
        stack1.clear();
    }
    return stack2.getFirst();
}
// Return whether the queue is empty.
public boolean empty() {
    return stack1.isEmpty() && stack2.isEmpty();
}