2171. EK求最大流

https://www.acwing.com/problem/content/submission/code_detail/11759361/

AcWing/2171/2171.cpp

@@ -0,0 +1,77 @@
+#include <cstring>
+#include <iostream>
+#include <limits>
+#include <queue>
+
+using std::cin;
+using std::cout;
+const char endl = '\n';
+
+const int N = 1005,
+          M = 10005;
+
+int n, m, s, t, ans;
+
+// Graph
+int idx, head[N], ver[M << 1], edge[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++;
+}
+
+// Edmonds-Karp
+int d[N], pre[N];
+bool vis[N];
+
+bool bfs() {
+    memset(vis, 0x00, sizeof(vis));
+    std::queue<int> q;
+    q.push(s);
+    vis[s] = true;
+    d[s] = std::numeric_limits<int>::max();
+    while (!q.empty()) {
+        int x = q.front();
+        q.pop();
+        for (int i = head[x]; ~i; i = next[i]) {
+            if (edge[i]) {
+                int y = ver[i];
+                if (vis[y]) continue;
+                d[y] = std::min(d[x], edge[i]);
+                pre[y] = i;
+                q.push(y);
+                vis[y] = true;
+                if (y == t) return true;
+            }
+        }
+    }
+    return false;
+}
+
+void update() {
+    int x = t;
+    while (x != s) {
+        int i = pre[x];
+        edge[i] -= d[t];
+        edge[i ^ 1] += d[t];
+        x = ver[i ^ 1];
+    }
+    ans += d[t];
+}
+
+int main() {
+    std::ios::sync_with_stdio(false);
+    memset(head, 0xff, sizeof(head));
+    cin >> n >> m >> s >> t;
+    for (int i = 1; i <= m; i++) {
+        int u, v, w;
+        cin >> u >> v >> w;
+        add(u, v, w);
+        add(v, u, 0);
+    }
+    while (bfs()) update();
+    cout << ans << endl;
+    return 0;
+}