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();}}