uva 10318 Security Panel (dfs+剪枝)

uva.onlinejudge.org/index.php?option=com_onlinejudge&Itemid=8&page=show_problem&problem=1259

　　压缩Panel的全部状态，预处理出每个位置的变化，然后dfs搜索全部情况。不过这样是会超时的，所以我在搜索的过程中加上剪枝，预判出之前的状态是否能够使得最后整个Panel充满，如果不能就尽早跳出搜索。加上小小的一个剪枝，速度提升估计了不下1000倍，由原来的超时，最后只要0.008s就完成了！

代码如下：

#define REP(i, n) for (int i = 0; i < (n); i++)
#define REP_1(i, n) for (int i = 1; i <= (n); i++)

const int dir[9][2] = { {-1, -1}, {-1, 0}, {-1, 1}, {0, -1}, {0, 0}, {0, 1}, {1, -1}, {1, 0}, {1, 1}};
char buf[3][4];
LL pat[25], ok[6];

void pntPat(LL n) {
    printf("~~~~~ %lld ~~~~~~\n", n);
    DEC(i, 63, 0) {
        if (n & (1ll << i)) putchar('1');
        else putchar('0');
        if ((i & 7) == 0) puts("");
    }
}

void makePat(int n, int m) {
    int t = (1 << m) - 1 << 1;
    REP(i, n + 1) {
        ok[i] = 0;
        REP(j, i) {
            ok[i] |= t;
            ok[i] <<= 8;
        }
//        pntPat(ok[i]);
    }
    REP(i, n) {
        REP(j, m) {
            int id = i * m + j;
            pat[id] = 0;
            REP(k, 9) {
                int x = dir[k][0] + i + 1, y = dir[k][1] + j + 1;
                if (buf[k / 3][k % 3] == '*') pat[id] |= 1ll << x * 8 + y;
            }
//            cout << id << endl;
//            pntPat(pat[id]);
        }
    }
}

int rec[30], cnt;

bool dfs(LL cur, int pos, int end, int n, int m) {
    if ((cur & ok[n]) == ok[n]) return true;
    if (pos >= end) return false;
    int t = pos / m - 1;
//    cout << pos << ends << m << ends << t << endl;
    if (t > 0 && (cur & ok[t]) != ok[t]) return false;
    if (dfs(cur, pos + 1, end, n, m)) return true;
    rec[cnt++] = pos + 1;
    if (dfs(cur ^ pat[pos], pos + 1, end, n, m)) return true;
    cnt--;
    return false;
}

void work(int n, int m) {
    makePat(n, m);
    cnt = 0;
    if (dfs(0, 0, n * m, n, m)) {
        REP(i, cnt) {
            if (i) putchar(' ');
            printf("%d", rec[i]);
        }
        puts("");
    } else {
        puts("Impossible.");
    }
}

int main() {
//    freopen("in", "r", stdin);
    int n, m, cas = 1;
    while (~scanf("%d%d", &n, &m) && (n || m)) {
        REP(i, 3) scanf("%s", buf[i]);
        printf("Case #%d\n", cas++);
        work(n, m);
    }
    return 0;
}

——written by Lyon