225.implement-stack-using-queues.cpp

/*
 * @lc app=leetcode id=225 lang=cpp
 *
 * [225] Implement Stack using Queues
 *
 * https://leetcode.com/problems/implement-stack-using-queues/description/
 *
 * algorithms
 * Easy (37.94%)
 * Total Accepted:    119.8K
 * Total Submissions: 315.4K
 * Testcase Example:  '["MyStack","push","push","top","pop","empty"]\n[[],[1],[2],[],[],[]]'
 *
 * Implement the following operations of a stack using queues.
 *
 *
 * push(x) -- Push element x onto stack.
 * pop() -- Removes the element on top of the stack.
 * top() -- Get the top element.
 * empty() -- Return whether the stack is empty.
 *
 *
 * Example:
 *
 *
 * MyStack stack = new MyStack();
 *
 * stack.push(1);
 * stack.push(2);
 * stack.top();   // returns 2
 * stack.pop();   // returns 2
 * stack.empty(); // returns false
 *
 * Notes:
 *
 *
 * You must use only standard operations of a queue -- which means only push to
 * back, peek/pop from front, size, and is empty operations are valid.
 * Depending on your language, queue may not be supported natively. You may
 * simulate a queue by using a list or deque (double-ended queue), as long as
 * you use only standard operations of a queue.
 * You may assume that all operations are valid (for example, no pop or top
 * operations will be called on an empty stack).
 *
 *
 */
#include <queue>
using namespace std;

class MyStack
{
  private:
    queue<int> q;
    int t;

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

    /** Push element x onto stack. */
    void push(int x)
    {
        q.push(x);
        t = x;
    }

    /** Removes the element on top of the stack and returns that element. */
    int pop()
    {
        int move = q.size();
        int result;
        while (move > 0)
        {
            result = q.front();
            q.pop();
            if (move == 2)
                t = result;
            if (move != 1)
                q.push(result);
            move--;
        }
        return result;
    }

    /** Get the top element. */
    int top()
    {
        return t;
    }

    /** Returns whether the stack is empty. */
    bool empty()
    {
        return q.empty();
    }
};

/**
 * Your MyStack object will be instantiated and called as such:
 * MyStack obj = new MyStack();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.top();
 * bool param_4 = obj.empty();
 */