Leetcode题解 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.

维护两个堆栈，一个只可以进，一个只可以出，百度15年测试实习生电话面试题。（同学面试遇到的）

class MyQueue {     private Stack<Integer> mStack1 = new Stack<Integer>();     private Stack<Integer> mStack2 = new Stack<Integer>();    // Push element x to the back of queue.    public void push(int x) {        mStack1.push(x);    }    // Removes the element from in front of queue.    public void pop() {        if (!empty()) {                if (mStack2.size() != 0) {                    mStack2.pop();                } else {                    while (mStack1.size() > 0) {                        mStack2.push(mStack1.pop());                    }                    mStack2.pop();                }            }    }    // Get the front element.    public int peek() {        if (!empty()) {                if (mStack2.size() != 0) {                    return mStack2.peek();                } else {                    while (mStack1.size() > 0) {                        mStack2.push(mStack1.pop());                    }                    return mStack2.peek();                }            }            return -1;    }    // Return whether the queue is empty.    public boolean empty() {        return mStack1.size() == 0 && mStack2.size() == 0;    }}