欢迎访问我的剑指offer（第二版）题解目录哦
对应的Leetcode题目可点击leetcode 232. Implement Queue using Stacks。

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.

Example:

MyQueue queue = new MyQueue();

queue.push(1);
queue.push(2);  
queue.peek();  // returns 1
queue.pop();   // returns 1
queue.empty(); // returns false

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).

算法设计

使用两个栈，一个栈s1用来负责模拟队列的压入元素操作，一个栈s2负责模拟队列的弹出元素操作。当需要向队列中压入元素时，直接向栈s1中压入这个元素。当需要从队列弹出队首元素时，首先检查栈s2是否为空，如果是空，则将栈s1中所有元素逐个弹出到栈s2中，然后弹出s2栈顶元素即可；如果不为空，直接弹出s2栈顶元素。

C++代码

class MyQueue {
  public:
    /** Initialize your data structure here. */
    stack<int> s1, s2;
    MyQueue() {}

    /** Push element x to the back of queue. */
    void push(int x) { s1.push(x); }

    /** Removes the element from in front of queue and returns that element. */
    int pop() {
        move_elements();
        int x = s2.top();
        s2.pop();
        return x;
    }

    /** Get the front element. */
    int peek() {
        move_elements();
        return s2.top();
    }

    /** Returns whether the queue is empty. */
    bool empty() { return s1.empty() and s2.empty(); }

    void move_elements() {
        if (s2.empty()) {
            while (not s1.empty()) {
                s2.push(s1.top());
                s1.pop();
            }
        }
    }
};

/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue* obj = new MyQueue();
 * obj->push(x);
 * int param_2 = obj->pop();
 * int param_3 = obj->peek();
 * bool param_4 = obj->empty();
 */