ZOJ 1002 Fire Net

深搜題，和八皇后類似，每一個節點，有兩個子節點，即放與不放 炮臺。

#include <stdio.h>
#include <string.h>
#define MAXN (10+5)

int N,amount,MAX;
char map[MAXN][MAXN],att[MAXN][MAXN];

int max(int x,int y)
{
    return x>y? x:y;
}

int isvalid( int cloumn,int row )
{
    int i;
    for( i= cloumn -1 ; i>=0 ;i-- )
    {
        if( att[i][row] == '@' ) return 0;
        if( att[i][row] == 'X' ) break;
    }
    for( i= row -1 ; i>=0 ;i-- )
    {
        if( att[cloumn][i] == '@' ) return 0;
        if( att[cloumn][i] == 'X' ) break;
    }
    return 1;
}

void DFS(int cloumn,int row)
{
	int next_cloumn,next_row;
	
    if( cloumn == N) {	MAX=max(MAX,amount);return ;}
	
	if( row+1 == N ) {	next_cloumn = cloumn+1;	next_row = 0;}
	else {	next_cloumn = cloumn;	next_row = row+1;	}
	
	if( map[cloumn][row] == 'X' ){	
		att[cloumn][row] ='X'; 
		DFS(next_cloumn,next_row);
	}
	else
	{
		if(  isvalid( cloumn, row ) )
		{
			att[cloumn][row] = '@';
			amount++;
			DFS(next_cloumn,next_row);
			amount--;
		}
		att[cloumn][row] = '.';
		DFS( next_cloumn, next_row );
	}
}

int main()
{
    int i;
    while( scanf("%d",&N) && N!=0)
    {
        MAX=0;  amount=0;
        memset( att,0,sizeof(att) );
        for(i=0;i<N;i++)
            scanf( "%s",map[i] );
        DFS(0,0);
        printf("%d\n",MAX);
    }
    return 0;
}