队列-Implement Queue using Stacks（用栈实现队列）

题目描述：

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). 

思考：

因为你只能使用栈在栈顶入栈（push）和出栈（pop),考虑使用两个栈，一个入栈一个出栈，当要从准队列里面拿出元素的时候，需要把入栈的元素反向放进出栈，两个栈的状态为同一时间两个栈不能保留同一个元素。

代码（java）：

static class MyQueue {Stack<Integer>  inStack = new Stack<Integer>();Stack<Integer> outStack = new Stack<Integer>();// Push element x to the back of queue.public void push(int x) {inStack.push(x);}// Removes the element from in front of queue.public void pop() {if(!outStack.empty()){outStack.pop();}else{//防止入栈和出栈有重复的相同元素，所以需要遍历完while(!inStack.empty()){outStack.push(inStack.pop()); }outStack.pop();}}// Get the front element.public int peek() {if(!outStack.empty()){return outStack.peek();}else{while(!inStack.empty()){outStack.push(inStack.pop()); }return outStack.peek();}}// Return whether the queue is empty.public boolean empty() {return outStack.empty()&&inStack.empty();}}