HDU 1045 Fire Net(DFS)

Posted 2020-07-01 镜中花/水中月

Fire Net

Problem Description

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. 

 

Input

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. 

 

Output

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

 

Meaning

在n*n的地图上防止尽可能多的blockhouse（碉堡），条件是碉堡不能看见另一个碉堡，然后地图上是有Fire Net的，可以隔开两个碉堡。

 

Answer

完全暴力DFS，就是写那个check函数有点烦。和经典的dfs不同之处是他的参数不是x和y，而是depth（深度）。每次搜索完成后（找不到能放置碉堡的地方），比较深度和ans的大小。

 

#include <iostream>
#include <cstdio>
#include <vector>
#include <set>
#include <algorithm>
using namespace std;
char mp[120][120];
int n,ans;
bool check(int x,int y)
{
    int i;
    if(mp[x][y]!=‘.‘) return false;
    if(x>=0)//up
        for(i=x; i>=0; i--)
            if(mp[i][y]==‘X‘)
                break;
            else if(mp[i][y]==‘1‘)
                return false;
    if(x<n)//down
        for(i=x; i<n; i++)
            if(mp[i][y]==‘X‘)
                break;
            else  if(mp[i][y]==‘1‘)
                return false;
    if(y>=0)//left
        for(i=y; i>=0; i--)
            if(mp[x][i]==‘X‘)
                break;
            else if(mp[x][i]==‘1‘)
                return false;
    if(y<n)//right
        for(i=y; i<n; i++)
            if(mp[x][i]==‘X‘)
                break;
            else if(mp[x][i]==‘1‘)
                return false;
    return true;
}
void dfs(int m)
{
    int i,j;
    for(i=0;i<n;i++)
    {
        for(j=0;j<n;j++)
        {
            if(check(i,j))
            {
                mp[i][j]=‘1‘;
                dfs(m+1);
                mp[i][j]=‘.‘;
            }
        }
    }
    ans=max(ans,m);
}
int main()
{
    while(cin>>n&&n)
    {
        ans=0;
        for(int i=0; i<n; i++)
            cin>>mp[i];
        dfs(0);
        printf("%d\n",ans);
    }
    return 0;
}

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