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。

 1 #include <cstdio>

 2 #include <cstring>

 3 #include <cstdlib>

 4 

 5 char map[8][8];

 6 int n;

 7 int max;

 8 

 9 bool check( int x, int y )

10 {

11     for ( int i = x - 1; i > 0; i-- ) if ( map[i][y] == '*' ) return false;

12         else if ( map[i][y] == 'X' ) break;

13 

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

15         else if ( map[i][y] == 'X' ) break;

16 

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

18         else if ( map[x][j] == 'X' ) break;

19 

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

21         else if ( map[x][j] == 'X' ) break;

22 

23     return true;

24 }

25 

26 void DFS( int x, int y, int sum )

27 {

28  //   printf( "x = %d, y = %d, sum = %d\n", x, y, sum );

29     if ( x == n && y == n )

30     {

31         if ( map[x][y] != 'X' && check( x, y ) ) ++sum;

32         if ( sum > max ) max = sum;

33         return;

34     }

35 

36     if ( map[x][y] == '.' && check( x, y ) )

37     {

38         for ( int i = x; i <= n; i++ )

39             for ( int j = 1; j <= n; j++ )

40             {

41                 if ( i == x && j == y ) continue;

42                 map[x][y] = '*';

43                 DFS( i, j, sum + 1 );

44                 map[x][y] = '.';

45             }

46     }

47     return;

48 }

49 

50 int main()

51 {

52   //  freopen( "s.out", "w", stdout );

53     while ( scanf( "%d", &n ), n )

54     {

55         memset( map, 'X', sizeof(map) );

56         for ( int i = 0; i < 8; i++ )

57             map[i][7] = '\0';

58 

59         for ( int i = 1; i <= n; i++ )

60         {

61             getchar();

62             for ( int j = 1; j <= n; j++ )

63                 map[i][j] = getchar();

64         }

65 

66         max = 0;

67         for ( int i = 1; i <= n; i++ )

68             for ( int j = 1; j <= n; j++ )

69                 if ( map[i][j] == '.' )  DFS( i, j, 0 );

70         printf( "%d\n", max );

71     }

72     return 0;

73 }

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