ZJU1002-Fire Net（dfs）

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

思路：直接dfs即可，（1）如果本单元格能够放置碉堡，则放置，crt加1；该方案递归完毕时，要将该单元格恢复过来，以便计算下一个方案。（2）本单元格不放置碉堡，进行方案递归。

ps:注意输入时回车符问题，用cin避免读取空格和回车符；

代码：

#include <bits/stdc++.h>using namespace std; const int max_n=4;char c[max_n][max_n];int MAX=0,n; bool C(int row,int col){for(int i=col-1;i>=0;i--)//判断row行上的合法性 {if(c[row][i]=='O')    return false;if(c[row][i]=='X')    break;}for(int i=row-1;i>=0;i--)//判断col列上的合法性 {if(c[i][col]=='O')    return false;if(c[i][col]=='X')    break;}return true;}void dfs(int index,int crt){if(index==n*n){MAX=max(MAX,crt);return ;}int x=index/n;int y=index%n;//如果本单元格可以放置碉堡 if(c[x][y]=='.'&&C(x,y)){c[x][y]='O';dfs(index+1,crt+1);c[x][y]='.';}dfs(index+1,crt);//本单元格不放置碉堡 }int main(){while(scanf("%d",&n)&&n){for(int i=0;i<n;i++)    for(int j=0;j<n;j++)        cin>>c[i][j];dfs(0,0);printf("%d\n",MAX);}return 0;}