#6007. 「网络流 24 题」方格取数

https://loj.ac/s/1418394

LibreOJ/6007/6007.cpp

@@ -0,0 +1,113 @@
+#include <cstring>
+#include <iostream>
+#include <limits>
+#include <queue>
+
+using std::cin;
+using std::cout;
+const char endl = '\n';
+
+const int N = 1005,
+          M = 100005;
+const int to[4][2] = {{0, 1}, {0, -1}, {1, 0}, {-1, 0}};
+
+int m, n, u, s, t, flow, ans, sum;
+
+// Graph
+int idx, head[N], edge[M << 1], ver[M << 1], next[M << 1];
+
+void add(int u, int v, int w) {
+    next[idx] = head[u];
+    ver[idx] = v;
+    edge[idx] = w;
+    head[u] = idx++;
+}
+
+// Dinic
+int d[N], cur[N];
+
+bool bfs() {
+    memset(d, 0x00, sizeof(d));
+    std::queue<int> q;
+    d[s] = 1;
+    q.push(s);
+    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 (w && !d[v]) {
+                d[v] = d[u] + 1;
+                cur[v] = head[v];
+                if (v == t) return true;
+                q.push(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]) {
+        int v = ver[i],
+            w = edge[i];
+        if (w && d[v] == d[u] + 1) {
+            int k = dinic(v, std::min(limit - flow, w));
+            if (!k) d[v] = 0;
+            edge[i] -= k;
+            edge[i ^ 1] += k;
+            flow += k;
+        }
+    }
+
+    return flow;
+}
+
+int main() {
+    std::ios::sync_with_stdio(false);
+
+    memset(head, 0xff, sizeof(head));
+
+    cin >> n >> m;
+    s = 0, t = n * m + 1;
+
+    for (int i = 1; i <= n; i++) {
+        for (int j = 1; j <= m; j++) {
+            int w, u = (i - 1) * m + j;
+            cin >> w;
+            sum += w;
+
+            if ((i + j) & 1) {
+                add(s, u, w);
+                add(u, s, 0);
+
+                for (int k = 0; k < 4; k++) {
+                    int xx = i + to[k][0],
+                        yy = j + to[k][1];
+                    if (1 <= xx && xx <= n && 1 <= yy && yy <= m) {
+                        int v = (xx - 1) * m + yy;
+                        add(u, v, std::numeric_limits<int>::max());
+                        add(v, u, 0);
+                    }
+                }
+            } else {
+                add(u, t, w);
+                add(t, u, 0);
+            }
+        }
+    }
+
+    while (bfs()) {
+        while (flow = dinic(s, std::numeric_limits<int>::max())) ans += flow;
+    }
+
+    cout << sum - ans << endl;
+    return 0;
+}