DFS专题 Fire Net

这道题暴力秒过，以前在ZOJ上见过，当时不会，还以为不剪枝铁定超呢，结果纯暴力就过了。

# include <cstdio>

# include <cstring>



# define N 4 + 3



int n, ans;

char g[N][N];



bool check(int x, int y)

{

    int i = x;

    if (g[x][y] != '.') return false;

    while (i <= n)

    {

        if (g[i][y] == 'X') break;

        else if (g[i][y] == (i-1)*n+y) return false;

        else ++i;

    }

    i = x-1;

    while (i >= 1)

    {

        if (g[i][y] == 'X') break;

        else if (g[i][y] == (i-1)*n+y) return false;

        else --i;

    }

    i = y;

    while (i <= n)

    {

        if (g[x][i] == 'X') break;

        else if (g[x][i] == (x-1)*n+i) return false;

        else ++i;

    }

    i = y-1;

    while (i >= 1)

    {

        if (g[x][i] == 'X') break;

        else if (g[x][i] == (x-1)*n+i) return false;

        else --i;

    }

    return true;

}



void dfs(int x, int y, int cnt)

{

    int i = 1, j = 1;

    //int tmp = (x-1)*n + y + 1, i = (tmp-1)/n+1, j = (tmp-1)%n+1;  // 错误的剪枝，没有通过样例

    //if (tmp == n*n) ans = (cnt > ans ? cnt : ans);

    if (cnt > ans) ans = cnt;

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

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

    {

        if (check(i,j))

        {

            g[i][j] = (i-1)*n+j;

            dfs(i, j, cnt+1);

            g[i][j] = '.';

        }

    }



}



void init(void)

{

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

        scanf("%s", g[i]+1);

}



void solve(void)

{

    ans = 0;



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

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

    {

        if (g[i][j]=='.')

        {

            g[i][j] = (i-1)*n+j;

            dfs(i, j, 1);

            g[i][j] = '.';

        }

    }



    printf("%d\n", ans);

}



int main()

{

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

    {

        init();

        solve();

    }



    return 0;

}

/**/