Implement Queue using Stacks

解题思路：采用两个栈，实现队列，一个用于进栈S1，一个用于出栈和取头元素S2。进栈时必须将S2中元素全部加入S1中，出栈时必须将S1中的元素加入S2中，才可以保证先进先出。判断为空时，S1，S2均为空。

Java代码实现：

class MyQueue {   private Stack stack1;private Stack stack2;MyQueue(){stack1=new Stack();stack2=new Stack();}    // Push element x to the back of queue.    public void push(int x) {       if(stack2.isEmpty()) stack1.push(x);       else {       while(!stack2.isEmpty()){       stack1.push(stack2.peek());       stack2.pop();       }       stack1.push(x);       }    }    // Removes the element from in front of queue.    public void pop() {        if(stack1.isEmpty()) {            if(!stack2.isEmpty())                stack2.pop();        }        else {        while(!stack1.isEmpty()){        stack2.push(stack1.peek());        stack1.pop();        }        stack2.pop();        }    }    // Get the front element.    public int peek() {    if(stack1.isEmpty()) {        if(!stack2.isEmpty())            return (int) stack2.peek();        return -1;    }        else {        while(!stack1.isEmpty()){        stack2.push(stack1.peek());        stack1.pop();        }        return (int) stack2.peek();        }    }    // Return whether the queue is empty.    public boolean empty() {        if(stack1.isEmpty()&&stack2.isEmpty()) return true;        else return false;    }}

原题地址：https://leetcode.com/problems/implement-queue-using-stacks/