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OI-codes/BZOJ/1189/1189.cpp

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#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;
}