#117. 有源汇有上下界最小流

https://loj.ac/s/1544490

LibreOJ/117/117.cpp

@@ -0,0 +1,133 @@
+#include <iostream>
+#include <cstring>
+#include <fstream>
+#include <limits>
+#include <queue>
+
+using std::cin;
+using std::cout;
+const char endl = '\n';
+
+const int N = 5e4 + 5,
+          M = 125003 + 5;
+
+int n, m, s, t, ss, tt;
+int idx, head[N], ver[M << 2], edge[M << 2], next[M << 2];
+int f[N], cur[N], dist[N];
+
+void add(int u, int v, int w) {
+    ver[idx] = v;
+    edge[idx] = w;
+    next[idx] = head[u];
+    head[u] = idx++;
+}
+
+bool bfs() {
+    memset(dist, 0x00, sizeof(dist));
+
+    std::queue<int> q;
+
+    q.push(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.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]) {
+        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;
+}
+
+int main() {
+    std::ios::sync_with_stdio(false);
+    cin.tie(nullptr);
+
+    memset(head, 0xff, sizeof(head));
+
+    cin >> n >> m >> ss >> tt;
+
+    for (int i = 1, u, v, a, b; i <= m; i++) {
+        cin >> u >> v >> a >> b;
+
+        add(u, v, b - a);
+        add(v, u, 0);
+
+        f[u] -= a, f[v] += a;
+    }
+
+    int sum = 0;
+    s = 0, t = n + 1;
+
+    for (int i = 1; i <= n; i++) {
+        if (f[i] > 0) {
+            add(s, i, f[i]);
+            add(i, s, 0);
+            sum += f[i];
+        } else if (f[i] < 0) {
+            add(i, t, -f[i]);
+            add(t, i, 0);
+        }
+    }
+
+    add(tt, ss, std::numeric_limits<int>::max());
+    add(ss, tt, 0);
+
+    int res = 0, flow;
+    while (bfs()) {
+        while (flow = dinic(s, std::numeric_limits<int>::max())) res += flow;
+    }
+
+    if (res == sum) {
+        int x = edge[idx - 1];
+        edge[idx - 1] = edge[idx - 2] = 0;
+
+        s = tt, t = ss;
+
+        int res = 0, flow;
+        while (bfs()) {
+            while (flow = dinic(s, std::numeric_limits<int>::max())) res += flow;
+        }
+
+        cout << x - res << endl;
+    } else {
+        cout << "please go home to sleep" << endl;
+    }
+
+    return 0;
+}