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).

两个队列实现一个堆栈。

class MyQueue {
　　private Queue<Integer> queue1 = new LinkedList<>();
　　private Queue<Integer> queue2 = new LinkedList<>();

　　private void shift() {
　　　　while(!queue1.isEmpty()) {
　　　　　　queue2.offer(queue1.poll());
　　　　}
　　}

　　// Push element x to the back of queue.
　　public void push(int x) {
　　　　queue1.offer(x);
　　}

　　// Removes the element from in front of queue.
　　public void pop() {
　　　　if (queue2.isEmpty()) {
　　　　　　shift();
　　　　}
　　　　queue2.poll();
　　}

　　// Get the front element.
　　public int peek() {
　　　　if (queue2.isEmpty()) {
　　　　　　shift();
　　　　}
　　　　return queue2.peek();
　　}

　　// Return whether the queue is empty.
　　public boolean empty() {
　　　　return queue1.isEmpty() && queue2.isEmpty();
　　}
}