网络流Ek算法

struct Edge {
    int from, to, cap, flow;
    Edge (int u, int v, int c, int f) : from(u), to(v), cap(c), flow(f) { }
};

struct EdmondsKarp {
    int n, m;
    vector<Edge> edges;
    vector<int>  G[maxn];
    int a[maxn];
    int p[maxn];
    
    void init(int n) {
        for (int i=0; i<n; ++i) G[i].clear();
        edges.clear();
    }
    
    void add(int u, int v , int val) {
        edges.push_back(Edge(u, v, val, 0));
        edges.push_back(Edge(u, v, 0, 0));
        m = edges.size();
        G[u].push_back(m-2);
        G[v].push_back(m-1);
    }
    
    int Maxflow(int s, int t) {
        int flow = 0;
        while (1) {
            memset(a, 0, sizeof(a));
            queue<int> Q;
            Q.push(s);
            a[s] = inf;
            while (!Q.empty()) {
                int x = Q.front(); Q.pop();
                for (int i=0; i<G[x].size(); ++i) {
                    Edge &e = edges[G[x][i]];
                    if (!a[e.to] && e.cap > e.flow) {
                        p[e.to] = G[x][i];
                        a[e.to] = min(a[x], e.cap - e.flow);
                        Q.push(e.to);
                    }
                }
                if (a[t]) break;
            }
            if (!a[t]) break;
            for (int u = t; u!=s; u =edges[p[u]].from) {
                edges[p[u]].flow += a[t];
                edges[p[u]^1].flow -= a[t];
            }
            flow += a[t];
        }
        return flow;
    }
}Ek;