最大流

struct Edge

{

    int from, to, cap, flow;

    Edge(int u, int v, int c, int f)

        : from(u), to(v), cap(c), flow(f) {}

};



const int maxn = 4096;

const int INF = 0x3f3f3f3f;

struct Dinic

{

    int n, m, s, t;      //结点数，边数（包括反向弧），源点编号和汇点编号

    vector<Edge> edges;  //边表。edge[e]和edge[e^1]互为反向弧

    vector<int> G[maxn]; //邻接表，G[i][j]表示节点i的第j条边在e数组中的序号

    bool vis[maxn];      //BFS使用

    int d[maxn];         //从起点到i的距离

    int cur[maxn];       //当前弧下标

    void init(int n)

    {

        this->n = n;

        for (int i = 0; i < n; i++) G[i].clear();

        edges.clear();

    }

    void AddEdge(int from, int to, int cap)

    {

        edges.emplace_back(from, to, cap, 0);

        edges.emplace_back(to, from, 0, 0);

        m = edges.size();

        G[from].push_back(m - 2);

        G[to].push_back(m - 1);

    }

    bool BFS()

    {

        memset(vis, 0, sizeof(vis));

        memset(d, 0, sizeof(d));

        queue<int> q;

        q.push(s);

        d[s] = 0;

        vis[s] = 1;

        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 (!vis[e.to] && e.cap > e.flow)

                {

                    vis[e.to] = 1;

                    d[e.to] = d[x] + 1;

                    q.push(e.to);

                }

            }

        }

        return vis[t];

    }

    int DFS(int x, int a)

    {

        if (x == t || a == 0) return a;

        int flow = 0, f;

        for (int& i = cur[x]; i < G[x].size(); i++)

        {

            //从上次考虑的弧

            Edge& e = edges[G[x][i]];

            if (d[x] + 1 == d[e.to] && (f = DFS(e.to, min(a, e.cap - e.flow))) > 0)

            {

                e.flow += f;

                edges[G[x][i] ^ 1].flow -= f;

                flow += f;

                a -= f;

                if (a == 0) break;

            }

        }

        return flow;

    }

    int Maxflow(int s, int t)

    {

        this->s = s;

        this->t = t;

        int flow = 0;

        while (BFS())

        {

            memset(cur, 0, sizeof(cur));

            flow += DFS(s, INF);

        }

        return flow;

    }

} gao;