232.implement-queue-using-stacks.cpp

/*
 * @lc app=leetcode id=232 lang=cpp
 *
 * [232] Implement Queue using Stacks
 *
 * https://leetcode.com/problems/implement-queue-using-stacks/description/
 *
 * algorithms
 * Easy (41.99%)
 * Total Accepted:    137.7K
 * Total Submissions: 327.7K
 * Testcase Example:  '["MyQueue","push","push","peek","pop","empty"]\n[[],[1],[2],[],[],[]]'
 *
 * 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).
 *
 *
 */
#include <stack>
using namespace std;

class MyQueue
{
  private:
    stack<int> s;
    stack<int> out;
    void queueify()
    {
        if (out.empty())
        {
            while (!s.empty())
            {
                out.push(s.top());
                s.pop();
            }
        }
    }

  public:
    /** Initialize your data structure here. */
    MyQueue()
    {
    }

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

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

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

    /** Returns whether the queue is empty. */
    bool empty()
    {
        return s.empty() && out.empty();
    }
};

/**
 * 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();
 */