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).

 思路：想要使用堆栈实现队列，堆栈出一个再进一个相当于没进没出，所以我们采用两个堆栈实现队列，先进第一个堆栈，再把第一个堆栈的元素依次给另外一个堆栈，那么即将要进来的元素就到了第一个堆栈的栈底，刚好，再从堆栈2到堆栈1，和队列的顺序就保持一致了。

public class MyQueue {    Stack<Integer>stack1=new Stack<>();    Stack<Integer>stack2=new Stack<>    /** Initialize your data structure here. */

    public MyQueue() {
        
    }
    
    /** Push element x to the back of queue. */
    public void push(int x) {
        while(!stack1.isEmpty())
        {
        	stack2.push(stack1.pop());//堆栈1的元素依次给堆栈2
        }
        stack1.push(x);//x就成了1的栈底元素

        while(!stack2.isEmpty())
        {
        	stack1.push(stack2.pop());//再把2给1
        }
    }
    
    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
         return stack1.pop();//栈顶元素就是队列的第一个元素
    }
    
    /** Get the front element. */
    public int peek() {
                return stack1.peek();
    }
    
    /** Returns whether the queue is empty. */
    public boolean empty() {
        return stack1.isEmpty();
    }
}

　　堆栈和队列之间的转换和巧妙，大家可以尝试一下！