hdu 1045:Fire Net(DFS经典题)
Fire Net
Time Limit: 2000/1000 MS (Java/Others) Memory Limit: 65536/32768 K (Java/Others)
Total Submission(s): 5863 Accepted Submission(s): 3280
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.
.X..
....
XX..
.... XX
.X .X.
X.X
.X. ...
.XX
.XX ....
....
....
....
#include <iostream> using namespace std;
char a[][];
int cnt,n;
bool judge(int x,int y) //判断这一步可不可以走
{
if(a[x][y]=='X')
return ;
if(a[x][y]=='*')
return ;
int i;
//判断这一行上有无碉堡
for(i=y;i>=;i--){
if(a[x][i]=='*')
return true;
if(a[x][i]=='X')
break;
}
for(i=y;i<=n;i++){
if(a[x][i]=='*')
return true;
if(a[x][i]=='X')
break;
}
//判断这一列上有无碉堡
for(i=x;i>=;i--){
if(a[i][y]=='*')
return ;
if(a[i][y]=='X')
break;
}
for(i=x;i<=;i++){
if(a[i][y]=='*')
return ;
if(a[i][y]=='X')
break;
}
//可以走
return ;
}
void dfs(int cx,int cy,int cn)
{
if(cn>cnt) cnt=cn; //记录最大值
int x=-,y=-;
int i,j;
for(i=cx;i<=n;i++) //选择这一步的位置
for(j=;j<=n;j++){
if(i==cx && j<=cy)
continue;
if(judge(i,j))
continue;
x=i;y=j;
a[x][y] = '*';
dfs(x,y,cn+);
a[x][y] = '.';
}
if(x==- && y==-)
return ;
}
int main()
{
while(cin>>n){
if(n==) break;
int i,j;
for(i=;i<=n;i++) //输入地图
for(j=;j<=n;j++)
cin>>a[i][j];
cnt = ;
dfs(,,); //深搜
cout<<cnt<<endl;
}
return ;
}
Freecode : www.cnblogs.com/yym2013
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