P3191 [HNOI2007]紧急疏散EVACUATE

https://www.luogu.com.cn/record/99301150

Luogu/P3191/P3191.cpp

@@ -0,0 +1,211 @@
+#include <iostream>
+#include <algorithm>
+#include <cstring>
+#include <iterator>
+#include <queue>
+
+using std::cin;
+using std::cout;
+const char endl = '\n';
+
+const int N = 25,
+          M = N * N,
+          E = 3e5 + 5;
+const int INF = 0x3f3f3f3f;
+
+const int d[][2] = {{0, 1}, {1, 0}, {0, -1}, {-1, 0}};
+
+int n, m, s, t, cnt, f[M][M];
+char g[N][N];
+int idx, head[E], ver[E << 1], edge[E << 1], next[E << 1];
+int cur[E << 1], dist[E << 1];
+bool vis[E << 1], vis2[M][M];
+
+int id(int x, int y) {
+    return (x - 1) * m + y;
+}
+
+void add(int u, int v, int w) {
+    next[idx] = head[u];
+    ver[idx] = v;
+    edge[idx] = w;
+    head[u] = idx++;
+}
+
+bool bfs() {
+    std::fill(std::begin(dist), std::end(dist), 0);
+
+    std::queue<int> q;
+
+    q.emplace(s);
+    dist[s] = 1;
+    cur[s] = head[s];
+
+    while (!q.empty()) {
+        int u = q.front();
+        q.pop();
+
+        for (int i = head[u]; ~i; i = next[i]) {
+            int v = ver[i],
+                w = edge[i];
+
+            if (!dist[v] && w) {
+                dist[v] = dist[u] + 1;
+                cur[v] = head[v];
+
+                if (v == t) return true;
+
+                q.emplace(v);
+            }
+        }
+    }
+
+    return false;
+}
+
+int dinic(int u, int limit) {
+    if (u == t) return limit;
+
+    int flow = 0;
+
+    for (int i = cur[u]; ~i && flow < limit; i = next[i]) {
+        cur[u] = i;
+
+        int v = ver[i],
+            w = edge[i];
+
+        if (dist[v] == dist[u] + 1 && w) {
+            int k = dinic(v, std::min(w, limit - flow));
+
+            if (!k) dist[v] = 0;
+
+            edge[i] -= k;
+            edge[i ^ 1] += k;
+            flow += k;
+        }
+    }
+
+    return flow;
+}
+
+bool check(int x) {
+    idx = 0;
+    std::fill(std::begin(head), std::end(head), -1);
+
+    for (int i = 1; i <= n; i++) {
+        for (int j = 1; j <= m; j++) {
+            if (g[i][j] == '.') {
+                add(s, id(i, j), 1);
+                add(id(i, j), s, 0);
+            } else if (g[i][j] == 'D') {
+                for (int k = 0; k < x; k++) {
+                    add(n * m + id(i, j) * x + k, t, 1);
+                    add(t, n * m + id(i, j) * x + k, 0);
+                }
+
+                for (int k = 0; k < x - 1; k++) {
+                    add(n * m + id(i, j) * x + k, n * m + id(i, j) * x + k + 1, INF);
+                    add(n * m + id(i, j) * x + k + 1, n * m + id(i, j) * x + k, 0);
+                }
+
+                for (int p = 1; p <= n; p++) {
+                    for (int q = 1; q <= m; q++) {
+                        if (g[p][q] == '.') {
+                            int d = f[id(p, q)][id(i, j)];
+
+                            if (d <= x) {
+                                add(id(p, q), n * m + id(i, j) * x + d - 1, INF);
+                                add(n * m + id(i, j) * x + d - 1, id(p, q), 0);
+                            }
+                        }
+                    }
+                }
+            }
+        }
+    }
+
+    int res = 0;
+
+    while (bfs()) {
+        while (int flow = dinic(s, INF)) res += flow;
+    }
+
+    return res == cnt;
+}
+
+int main() {
+    std::ios::sync_with_stdio(false);
+    cin.tie(nullptr);
+
+    memset(f, 0x3f, sizeof(f));
+
+    cin >> n >> m;
+
+    s = 0, t = 10000;
+
+    for (int i = 1; i <= n; i++) {
+        for (int j = 1; j <= m; j++) {
+            cin >> g[i][j];
+
+            if (g[i][j] == '.') cnt++;
+        }
+    }
+
+    for (int i = 1; i <= n; i++) {
+        for (int j = 1; j <= m; j++) {
+            if (g[i][j] == 'D') {
+                memset(vis2, 0x00, sizeof(vis2));
+
+                f[id(i, j)][id(i, j)] = 0;
+
+                std::queue<std::pair<int, int>> q;
+
+                q.emplace(i, j);
+
+                while (!q.empty()) {
+                    int x = q.front().first,
+                        y = q.front().second;
+                    q.pop();
+
+                    for (auto d : d) {
+                        int xx = x + d[0],
+                            yy = y + d[1];
+
+                        if (xx < 0 || xx > n || yy < 0 || yy > m || g[xx][yy] != '.' || vis2[xx][yy]) continue;
+
+                        vis2[xx][yy] = true;
+
+                        if (f[id(xx, yy)][id(i, j)] > f[id(x, y)][id(i, j)] + 1) {
+                            f[id(xx, yy)][id(i, j)] = f[id(x, y)][id(i, j)] + 1;
+
+                            q.emplace(xx, yy);
+                        }
+                    }
+                }
+            }
+        }
+    }
+
+    int l = 0,
+        r = cnt,
+        res = -1;
+
+    while (l <= r) {
+        int mid = (l + r) >> 1;
+
+        if (check(mid)) {
+            r = mid - 1;
+            res = mid;
+        } else {
+            l = mid + 1;
+        }
+    }
+
+    if (res == -1) {
+        cout << "impossible" << endl;
+    } else {
+        cout << res << endl;
+    }
+
+    return 0;
+}