LeetCode-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).

这种题可以自己先跑几个样例就会发现是有规律可循的，主要思路是两个stack，一个stack1用于push，另外一个stack2用于pop，关键就在从stack1倒到stack2的过程，还是直接上代码吧，一目了然：

class MyQueue {        LinkedList<Integer> stack1 = new LinkedList<Integer>();    LinkedList<Integer> stack2 = new LinkedList<Integer>();        // Push element x to the back of queue.    public void push(int x) {        stack1.push(x);    }    // Removes the element from in front of queue.    public void pop() {       if (stack2.isEmpty()) {           while (!stack1.isEmpty()) {               stack2.push(stack1.pop());           }       }       stack2.pop();    }    // Get the front element.    public int peek() {    if (stack2.isEmpty()) {           while (!stack1.isEmpty()) {               stack2.push(stack1.pop());           }       }       return stack2.peek();    }    // Return whether the queue is empty.    public boolean empty() {        return stack1.isEmpty() && stack2.isEmpty();    }}