mirror of
https://git.sb/baoshuo/OI-codes.git
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212 lines
4.6 KiB
C++
212 lines
4.6 KiB
C++
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#include <iostream>
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#include <algorithm>
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#include <cstring>
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#include <iterator>
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#include <queue>
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using std::cin;
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using std::cout;
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const char endl = '\n';
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const int N = 25,
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M = N * N,
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E = 3e5 + 5;
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const int INF = 0x3f3f3f3f;
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const int d[][2] = {{0, 1}, {1, 0}, {0, -1}, {-1, 0}};
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int n, m, s, t, cnt, f[M][M];
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char g[N][N];
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int idx, head[E], ver[E << 1], edge[E << 1], next[E << 1];
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int cur[E << 1], dist[E << 1];
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bool vis[E << 1], vis2[M][M];
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int id(int x, int y) {
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return (x - 1) * m + y;
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}
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void add(int u, int v, int w) {
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next[idx] = head[u];
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ver[idx] = v;
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edge[idx] = w;
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head[u] = idx++;
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}
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bool bfs() {
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std::fill(std::begin(dist), std::end(dist), 0);
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std::queue<int> q;
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q.emplace(s);
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dist[s] = 1;
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cur[s] = head[s];
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while (!q.empty()) {
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int u = q.front();
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q.pop();
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for (int i = head[u]; ~i; i = next[i]) {
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int v = ver[i],
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w = edge[i];
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if (!dist[v] && w) {
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dist[v] = dist[u] + 1;
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cur[v] = head[v];
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if (v == t) return true;
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q.emplace(v);
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}
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}
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}
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return false;
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}
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int dinic(int u, int limit) {
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if (u == t) return limit;
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int flow = 0;
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for (int i = cur[u]; ~i && flow < limit; i = next[i]) {
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cur[u] = i;
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int v = ver[i],
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w = edge[i];
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if (dist[v] == dist[u] + 1 && w) {
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int k = dinic(v, std::min(w, limit - flow));
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if (!k) dist[v] = 0;
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edge[i] -= k;
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edge[i ^ 1] += k;
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flow += k;
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}
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}
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return flow;
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}
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bool check(int x) {
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idx = 0;
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std::fill(std::begin(head), std::end(head), -1);
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for (int i = 1; i <= n; i++) {
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for (int j = 1; j <= m; j++) {
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if (g[i][j] == '.') {
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add(s, id(i, j), 1);
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add(id(i, j), s, 0);
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} else if (g[i][j] == 'D') {
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for (int k = 0; k < x; k++) {
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add(n * m + id(i, j) * x + k, t, 1);
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add(t, n * m + id(i, j) * x + k, 0);
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}
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for (int k = 0; k < x - 1; k++) {
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add(n * m + id(i, j) * x + k, n * m + id(i, j) * x + k + 1, INF);
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add(n * m + id(i, j) * x + k + 1, n * m + id(i, j) * x + k, 0);
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}
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for (int p = 1; p <= n; p++) {
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for (int q = 1; q <= m; q++) {
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if (g[p][q] == '.') {
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int d = f[id(p, q)][id(i, j)];
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if (d <= x) {
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add(id(p, q), n * m + id(i, j) * x + d - 1, INF);
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add(n * m + id(i, j) * x + d - 1, id(p, q), 0);
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}
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}
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}
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}
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}
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}
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}
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int res = 0;
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while (bfs()) {
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while (int flow = dinic(s, INF)) res += flow;
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}
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return res == cnt;
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}
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int main() {
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std::ios::sync_with_stdio(false);
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cin.tie(nullptr);
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memset(f, 0x3f, sizeof(f));
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cin >> n >> m;
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s = 0, t = 10000;
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for (int i = 1; i <= n; i++) {
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for (int j = 1; j <= m; j++) {
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cin >> g[i][j];
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if (g[i][j] == '.') cnt++;
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}
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}
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for (int i = 1; i <= n; i++) {
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for (int j = 1; j <= m; j++) {
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if (g[i][j] == 'D') {
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memset(vis2, 0x00, sizeof(vis2));
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f[id(i, j)][id(i, j)] = 0;
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std::queue<std::pair<int, int>> q;
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q.emplace(i, j);
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while (!q.empty()) {
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int x = q.front().first,
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y = q.front().second;
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q.pop();
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for (auto d : d) {
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int xx = x + d[0],
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yy = y + d[1];
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if (xx < 0 || xx > n || yy < 0 || yy > m || g[xx][yy] != '.' || vis2[xx][yy]) continue;
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vis2[xx][yy] = true;
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if (f[id(xx, yy)][id(i, j)] > f[id(x, y)][id(i, j)] + 1) {
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f[id(xx, yy)][id(i, j)] = f[id(x, y)][id(i, j)] + 1;
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q.emplace(xx, yy);
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}
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}
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}
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}
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}
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}
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int l = 0,
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r = cnt,
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res = -1;
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while (l <= r) {
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int mid = (l + r) >> 1;
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if (check(mid)) {
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r = mid - 1;
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res = mid;
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} else {
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l = mid + 1;
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}
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}
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if (res == -1) {
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cout << "impossible" << endl;
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} else {
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cout << res << endl;
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}
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return 0;
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}
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