每天一个小算法（6）---- 通过两个栈实现一个队列

这个算法也很简单，定义两个栈m_aStack、m_bStack，m_aStack负责push()数据，m_bStack负责front()数据。

思路：每一次front()取数据都会检查一下m_bStack是否为空，为空则把m_aStack的所有数据pop()出来push()到m_bStack中。

因为STL里有stack，直接拿来用了，代码使用C++，在linux/g++下编译运行成功：

#include <stack>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>

class Fifo
{
public:
    const int size() const;
    const bool empty() const;
    const int front();
    void push(int element);
    void print() const;
private:
    std::stack<int> m_aStack;
    std::stack<int> m_bStack;
};

const int Fifo::size() const
{
    return m_aStack.size() + m_bStack.size();
}

const bool Fifo::empty() const
{
    return m_aStack.empty() & m_bStack.empty();
}

const int Fifo::front()
{
    if( empty() )
        return 0;

    if ( m_bStack.empty() )
    {
        while ( !m_aStack.empty() )
        {
            m_bStack.push(m_aStack.top());
            m_aStack.pop();
        }
    }

    int tmp = m_bStack.top();
    m_bStack.pop();
    return tmp;
}

void Fifo::push(int element)
{
    m_aStack.push(element);
}

void Fifo::print() const
{
    if ( empty() )
        return;

    std::stack<int> tAStack = m_aStack;
    std::stack<int> tBStack = m_bStack;

    printf("the elements of the m_aStack:	");
    while ( !tAStack.empty() )
    {
        printf("%d ", tAStack.top());
        tAStack.pop();        
    }
    printf("
");
    
    printf("the elements of the m_bStack:	");
    while ( !tBStack.empty() )
    {
        printf("%d ", tBStack.top());        
        tBStack.pop();        
    }
    printf("
");

}

int main(int argc, char const *argv[])
{
    Fifo fifo;
    srand((int)time(0));
    for ( int i=0; i< 100; ++i )
    {
        if ( (rand() & 1) == 0 )
        {
            int tmp = rand()%100;
            printf("
push %d to fifo
", tmp);
            fifo.push(tmp);
            fifo.print();
        }
        else
        {
            printf("
the front of the fifo: %d
", fifo.front());            
            fifo.print();
        }
    }

    return 0;
}