ZOJ 1002 Fire Net

题目链接：http://acm.zju.edu.cn/onlinejudge/showProblem.do?problemCode=1002

分析：类八皇后问题，简单DFS。

#include <cstdio>
#include <cstring>
#include <cstdlib>

char map[8][8];
int n;
int max;

bool check( int x, int y )
{
    for ( int i = x - 1; i > 0; i-- ) if ( map[i][y] == '*' ) return false;
        else if ( map[i][y] == 'X' ) break;

    for ( int i = x + 1; i <= n; i++ ) if ( map[i][y] == '*' ) return false;
        else if ( map[i][y] == 'X' ) break;

    for ( int j = y - 1; j >= 0; j-- ) if ( map[x][j] == '*' ) return false;
        else if ( map[x][j] == 'X' ) break;

    for ( int j = y + 1; j <= n; j++ ) if ( map[x][j] == '*' ) return false;
        else if ( map[x][j] == 'X' ) break;

    return true;
}

void DFS( int x, int y, int sum )
{
 //   printf( "x = %d, y = %d, sum = %d\n", x, y, sum );
    if ( x == n && y == n )
    {
        if ( map[x][y] != 'X' && check( x, y ) ) ++sum;
        if ( sum > max ) max = sum;
        return;
    }

    if ( map[x][y] == '.' && check( x, y ) )
    {
        for ( int i = x; i <= n; i++ )
            for ( int j = 1; j <= n; j++ )
            {
                if ( i == x && j == y ) continue;
                map[x][y] = '*';
                DFS( i, j, sum + 1 );
                map[x][y] = '.';
            }
    }
    return;
}

int main()
{
  //  freopen( "s.out", "w", stdout );
    while ( scanf( "%d", &n ), n )
    {
        memset( map, 'X', sizeof(map) );
        for ( int i = 0; i < 8; i++ )
            map[i][7] = '\0';

        for ( int i = 1; i <= n; i++ )
        {
            getchar();
            for ( int j = 1; j <= n; j++ )
                map[i][j] = getchar();
        }

        max = 0;
        for ( int i = 1; i <= n; i++ )
            for ( int j = 1; j <= n; j++ )
                if ( map[i][j] == '.' )  DFS( i, j, 0 );
        printf( "%d\n", max );
    }
    return 0;
}

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