N-Queens Problem（recursion）

//queens.h
#pragma once
const int maxboard = 30;
class Queens
{
public:
	Queens(int size);
	bool is_solved()const;
	void print();
	bool ungarded(int col)const;//优化后
	void insert(int col);
	void remove(int col);
	void solve_from(Queens &configuration);

private:
	int board_size;
	int count;
	bool col_free[maxboard];//一些优化操作
	bool upward_free[2 * maxboard - 1];
	bool downward_free[2 * maxboard - 1];
	int queen_in_row[maxboard];

};

//queens.cpp
#include"queens.h"
#include<iostream>
using namespace std;
bool Queens::is_solved()const{
	return count == board_size;
}

void Queens::print()
{
	for (int row = 0; row < board_size; row++) {
		for (int i = 0; i < queen_in_row[row]; i++)cout << " 0 ";
		cout << " 1 ";
		for (int i = queen_in_row[row] + 1; i < board_size; i++)cout << " 0 ";
		cout << endl;
	}
	cout <<"----------------------------------------------" <<endl;
}

bool Queens::ungarded(int col) const
{
	return (col_free[col] && upward_free[count + col] && downward_free[count - col + board_size - 1]);
}

void Queens::insert(int col)
{
	queen_in_row[count] = col;
	col_free[col] = false;
	upward_free[count + col] = false;
	downward_free[count - col + board_size - 1] = false;
	count++;
}

void Queens::remove(int col)
{
	count--;
	queen_in_row[count] = 0;
	col_free[col] = true;
	upward_free[count + col] = true;
	downward_free[count - col + board_size - 1] = true;
}

void Queens::solve_from(Queens & configuration)
{
	if (configuration.is_solved());//configuration.print();
	else {
		for (int col = 0; col < board_size; col++) {
			if (configuration.ungarded(col)) {
				configuration.insert(col);
				solve_from(configuration);
				configuration.remove(col);
			}
		}
	}
}

Queens::Queens(int size)
{
	board_size = size;
	count = 0;
	for (int i = 0; i < maxboard; i++)col_free[i] = true;
	for (int i = 0; i < 2 * maxboard - 1; i++)upward_free[i] = true;
	for (int i = 0; i < 2 * maxboard - 1; i++)downward_free[i] = true;
}

//main.cpp
#include"queens.h"
#include<iostream>
#include<Windows.h>
using namespace std;
void main() {
	while (1) {
		DWORD start_time = GetTickCount();
		{
			cout << "请输入需要计算的皇后数量（maxcount=30）。" << endl;
			int count;
			cin >> count;
			Queens configuration(count);
			configuration.solve_from(configuration);
		}
		DWORD end_time = GetTickCount();
		cout << "The run time is:" << (end_time - start_time) << "ms!" << endl;//输出运行时间
	}
}