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;
}

/**/