#1904. 【LOJ2239】「CQOI2014」危桥

https://sjzezoj.com/submission/68663

S2OJ/1904/1904.cpp

@@ -0,0 +1,147 @@
+#include <iostream>
+#include <algorithm>
+#include <iterator>
+#include <queue>
+
+using std::cin;
+using std::cout;
+const char endl = '\n';
+
+const int N = 55,
+          M = (N * N) << 1;
+const int INF = 0x3f3f3f3f;
+
+int n, s, t, a1, a2, an, b1, b2, bn;
+int idx, head[N], head2[N], ver[M << 1], edge[M << 1], edge2[M << 1], next[M << 1];
+int dis[N], cur[N];
+
+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(dis), std::end(dis), 0);
+
+    std::queue<int> q;
+
+    q.emplace(s);
+    dis[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 (w && !dis[v]) {
+                dis[v] = dis[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 (w && dis[v] == dis[u] + 1) {
+            int k = dinic(v, std::min(w, limit - flow));
+
+            if (!k) dis[v] = 0;
+
+            edge[i] -= k;
+            edge[i ^ 1] += k;
+            flow += k;
+        }
+    }
+
+    return flow;
+}
+
+int main() {
+    std::ios::sync_with_stdio(false);
+    cin.tie(nullptr);
+
+    while (cin >> n >> a1 >> a2 >> an >> b1 >> b2 >> bn) {
+        a1++, a2++, b1++, b2++;
+        s = 0, t = n + 1;
+
+        idx = 0;
+        std::fill(std::begin(head), std::end(head), -1);
+
+        for (int i = 1; i <= n; i++) {
+            for (int j = 1; j <= n; j++) {
+                char c;
+
+                cin >> c;
+
+                if (c == 'O') {
+                    add(i, j, 2);
+                    add(j, i, 0);
+                } else if (c == 'N') {
+                    add(i, j, INF);
+                    add(j, i, 0);
+                }
+            }
+        }
+
+        bool ans = true;
+
+        for (int _ = 0; _ < 2; _++) {
+            // Backup
+            int idx2 = idx;
+            std::copy(std::begin(head), std::end(head), std::begin(head2));
+            std::copy(std::begin(edge), std::end(edge), std::begin(edge2));
+
+            add(s, a1, an * 2);
+            add(a1, s, 0);
+
+            add(s, b1, bn * 2);
+            add(b1, s, 0);
+
+            add(a2, t, an * 2);
+            add(t, a2, 0);
+
+            add(b2, t, bn * 2);
+            add(t, b2, 0);
+
+            int res = 0;
+
+            while (bfs()) {
+                while (int flow = dinic(s, INF)) res += flow;
+            }
+
+            ans &= res == (an + bn) * 2;
+
+            std::swap(b1, b2);
+
+            // Restore backup
+            idx = idx2;
+            std::copy(std::begin(head2), std::end(head2), std::begin(head));
+            std::copy(std::begin(edge2), std::end(edge2), std::begin(edge));
+        }
+
+        cout << (ans ? "Yes" : "No") << endl;
+    }
+
+    return 0;
+}