LeetCode Implement Queue using Stacks

题目：

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).
• 就是使用一个双栈来实现一个队列，可以使用栈的一些个方法,例如push或者是top等方法。
  

题意：

代码：

class MyQueue {    Stack<Integer> stack1 = new Stack<Integer>();    Stack<Integer> stack2 = new Stack<Integer>();    // Push element x to the back of queue.    public void push(int x)     {        if(!stack1.isEmpty())            //这里的判断第一个栈是否为空            stack1.push(x);        else                                    //如果第一个栈为空，那么就要从第二个栈中的数据压入第一个栈中        {            while(!stack2.isEmpty())                stack1.push(stack2.pop());            stack1.push(x);        }    }    // Removes the element from in front of queue.    public void pop()     {        if(!stack2.isEmpty())           stack2.pop();        else        {            while(!stack1.isEmpty())                stack2.push(stack1.pop());            stack2.pop();        }    }    // Get the front element.    public int peek()     {        if(!stack2.isEmpty())           return stack2.peek();        else        {            while(!stack1.isEmpty())                stack2.push(stack1.pop());            return stack2.peek();        }    }    // Return whether the queue is empty.    public boolean empty()     {        return (stack1.isEmpty() && stack2.isEmpty());    }}