模板积累——网络流建图及dinic算法

#include 

typedef long long ll;

const int MAXN = 11000;
const int INF = 1e9;

struct Edge {
    int u, v;
    int cap, flow;
    Edge(int u_, int v_, int cap_, int flow_) : u(u_), v(v_), cap(cap_), flow(flow_) {}
};

int n, s, t;
std::vector edges;
std::vector g[MAXN];
bool visited[MAXN];
int depth[MAXN];
int current[MAXN];
	
void init() {
    edges.clear();
    for(int i = 0; i < n; ++i) g[i].clear();
}
void addEdge(int u, int v, int cap) {
    int index0 = edges.size(), index1 = index0 + 1;
    edges.push_back(Edge(u, v, cap, 0));
    edges.push_back(Edge(v, u, 0, 0));
    g[u].push_back(index0);
    g[v].push_back(index1);
}
bool bfs() {
    for(int i = 0; i < n; ++i) visited[i] = false, depth[i] = 0;
    std::queue q;
    visited[s] = true;
    depth[s] = 0;
    q.push(s);
    do {
        int u = q.front();
        q.pop();
        for(int i = 0; i < int(g[u].size()); ++i) {
            Edge& e = edges[g[u][i]];
            int v = e.v;
            if(visited[v] || e.flow >= e.cap) continue;
            visited[v] = true;
            depth[v] = depth[u] + 1;
            q.push(v);
        }
    } while(!q.empty());
    return visited[t];
}
int dfs(int u, int flow) {
    if(u == t || !flow) return flow;
    int result = 0;
    for(int& i = current[u]; i < int(g[u].size()); ++i) {
        Edge& e = edges[g[u][i]];
        int v = e.v;
        if(depth[v] != depth[u] + 1) continue;
        int f = dfs(v, std::min(flow, e.cap - e.flow));
        if(!f) continue;
        Edge& r = edges[g[u][i] ^ 1];
        e.flow += f;
        r.flow -= f;
        result += f;
        flow -= f;
        if(!flow) break;
    }
    return result;
}
int dinic() {
    int result = 0;
    while(bfs()) {
        for(int i = 0; i < n; ++i) current[i] = 0;
        result += dfs(s, INF);
    }
    return result;
}

int main() {
    int n, m;
    std::scanf("%d%d%d%d", &n, &m, &s, &t);
    ::n = n;
    --s, --t;
    init();
    for(int i = 0; i < m; ++i) {
        int u, v;
        int w;
        std::scanf("%d%d%d", &u, &v, &w);
        --u, --v;
        addEdge(u, v, w);
    }
    int result = dinic();
    std::printf("%d\n", result);
}