**joj:1017 fire net 递归回溯的使用

 1017: Fire Net

---

Result TIME Limit MEMORY Limit Run Times AC Times JUDGE

	3s	8192K	1086	565	Standard

Suppose that we have a square city with straight streets. A map of a city is a square board with n rows and n columns, each representing a street or a piece of wall.

A blockhouse is a small castle that has four openings through which to shoot. The four openings are facing North, East, South, and West, respectively. There will be one machine gun shooting through each opening.

Here we assume that a bullet is so powerful that it can run across any distance and destroy a blockhouse on its way. On the other hand, a wall is so strongly built that can stop the bullets.

The goal is to place as many blockhouses in a city as possible so that no two can destroy each other. A configuration of blockhouses is legal provided that no two blockhouses are on the same horizontal row or vertical column in a map unless there is at least one wall separating them. In this problem we will consider small square cities (at most 4x4) that contain walls through which bullets cannot run through.

The following image shows five pictures of the same board. The first picture is the empty board, the second and third pictures show legal configurations, and the fourth and fifth pictures show illegal configurations. For this board, the maximum number of blockhouses in a legal configuration is 5; the second picture shows one way to do it, but there are several other ways.

 

Your task is to write a program that, given a description of a map, calculates the maximum number of blockhouses that can be placed in the city in a legal configuration.

The input file contains one or more map descriptions, followed by a line containing the number 0 that signals the end of the file. Each map description begins with a line containing a positive integer n that is the size of the city; n will be at most 4. The next n lines each describe one row of the map, with a '.' indicating an open space and an uppercase 'X' indicating a wall. There are no spaces in the input file.

For each test case, output one line containing the maximum number of blockhouses that can be placed in the city in a legal configuration.

 

Sample Input

4
.X..
....
XX..
....
2
XX
.X
3
.X.
X.X
.X.
3
...
.XX
.XX
4
....
....
....
....
0

 

Sample Output

5
1
5
2
4

 

 

 

/*
	利用回溯的方法来判断所有的可能
*/
#include <stdio.h>
#include <string.h>
int time = 0;
char con[4][4];
int num,Max;
bool check2()
{
	int	i , j;
	for( i = 0 ; i < time ; ++i )
		{
		for( j = 0 ; j < time ; ++j )
			{
			if( con[i][j] == 'f' )
				{
				int	row , col;
				for( row = i + 1 ; row < time && con [row][j] != 'X' ; ++row )
					if( con [row][j] == 'f' )
						return	false;
				for( col = j + 1 ; col < time && con [i][col] != 'X' ; ++col )
					if( con [i][col] == 'f' )
						return	false;
				}
			}
		}
	return	true;
}
void Rec3(int row,int col)	
{
	if(row >= time) //边界之一
	{
		if(num > Max)
			Max = num;
		return;
	}
	if(col >= time) //边界之二
	{		
		Rec3(row +1,0);//col 从0开始
		return ; //这个return 是必须的，要不然回退到此处时候，还会向下执行
	}	
	if(con[row][col] == '.')
	{
		//////////////////////////////////////////////////////////////////////////////////////////
		con[row][col] = 'f';
		num ++;		
		if(check2())		
			 Rec3(row,col + 1); /////////////////////////////////////////////////本区间的代码是回溯可以产生的关键
		num --;
		con[row][col] = '.';
		Rec3(row,col + 1);				
		///////////////////////////////////////////////////////////////////////////////////////////
	}
	else
	{
		Rec3(row,col +1);
	}

}
int main()
{
	memset(con,'\0',sizeof(con));
	freopen("in.txt","r",stdin);
	while(scanf("%d\n",&time) != EOF && time != 0){	
		for(int i=0;i<time;i++)
		{
			for(int j=0;j<time;j++)	
			{
				con[i][j] = (char)getchar();
				if(j == time -1)
					getchar();
			}	
			

				//scanf("%c",&con[i][j]);

		}				
		num = 0;
		Max =0;
		Rec(0,0);
		printf("%d\n",Max);
		
		memset(con,'\0',sizeof(con));
	}
	fclose(stdin);
	return 0;
}