图论板子

Prim

typedef pair <int, int> P;
int prim() {
    int ans = 0, cnt = 0;
    priority_queue <P, vector<P>, greater<P> > q;
    fill(d + 1, d + n + 1, INF);
    d[1] = 0;
    q.push(P(d[1], 1));
    while(!q.empty() && cnt < n) {
        int y = q.top().second;
        q.pop();
        if(vis[y]) continue;
        vis[y] = 1;
        cnt++;
        ans += d[y];
        for(int i = 0; i < v[y].size(); i++) {
            int t = v[y][i].t, w = v[y][i].w;
            if(w < d[t]) {
                d[t] = w;
                q.push(P(d[t], t));
            }
        }
    }
    if(cnt < n) ans = -1;
    return ans;
}

tarjan（缩点）

int dfn[M], low[M], z;
int col[M], c;
int st[M], it;
bool vis[M];
void tarjan(int i) {
    z++;
    dfn[i] = low[i] = z;
    it++;
    st[it] = i;
    vis[i] = 1;
    for(int j = 0; j < v[i].size(); j++) {
        int t = v[i][j];
        if(!dfn[t]) {
            tarjan(t);
            low[i] = min(low[i], low[t]);
        } else if(vis[t]) {
            low[i] = min(low[i], dfn[t]);
        }
    }
    if(low[i] == dfn[i]) {
        c++;
        while(st[it] != i) {
            col[st[it]] = cc;
            vis[st[it]] = 0;
            it--;
        }
        col[i] = c;
        vis[i] = 0;
        it--;
    }
}

Dinic

int st, ed, l[N], it[N];
struct edge{
    int t, w, r;
};
vector <edge> v[N];
void add(int f, int t, int w) {
    v[f].push_back({t, w, (int)v[t].size()});
    v[t].push_back({f, 0, (int)v[f].size() - 1});
}
void bfs() {
    for(int i = st; i <= ed; i++) l[i] = -1;
    queue <int> q;
    l[st] = 0;
    q.push(st);
    while(!q.empty()) {
        int x = q.front();
        q.pop();
        for(int i = 0; i < v[x].size(); i++) {
            int t = v[x][i].t, w = v[x][i].w;
            if(w > 0 && l[t] < 0) {
                l[t] = l[x] + 1;
                q.push(t);
            }
        }
    }
}
int dinic(int i, int fl) {
    if(i == ed) return fl;
    for(; it[i] < v[i].size(); it[i]++) {
        int t = v[i][it[i]].t, w = v[i][it[i]].w, r = v[i][it[i]].r;
        if(w > 0 && l[t] > l[i]) {
            int d = dinic(t, min(fl, w));
            if(d > 0) {
                v[i][it[i]].w -= d;
                v[t][r].w += d;
                return d;
            }
        }
    }
    return 0;
}
int main() {
    //...
    int flow = 0;
    for(;;) {
        bfs();
        if(l[ed] < 0) break;
        for(int i = st; i <= ed; i++) {
            it[i] = 0;
        }
        for(;;) {
            int f = dinic(st, INF);
            if(f <= 0) break;
            flow += f;
        }
    }
    return 0;
}

min_cost_flow (Primal-Dual)

void add(int f, int t, int w, int c) {
    v[f].push_back({t, w, c, (int)v[t].size()});
    v[t].push_back({f, 0, -c, (int)v[f].size() - 1});
}
int h[N], d[N], pre[N], id[N];
bool vis[N];
void spfa() {
    queue<int> q;
    fill(h, h + ed + 1, INF);
    h[st] = 0, vis[st] = 1;
    q.push(st);
    while (!q.empty()) {
        int x = q.front();
        q.pop();
        vis[x] = 0;
        for(int i = 0; i < v[x].size(); i++) {
            int t = v[x][i].t, w = v[x][i].w, c = v[x][i].c;
            if(w > 0 && h[t] > h[x] + c) {
                h[t] = h[x] + c;
                if(!vis[t]) {
                    vis[t] = 1;
                    q.push(t);
                }
            }
        }
    }
}
void dijkstra() {
    fill(d + 1, d + ed + 1, INF);
    d[st] = 0;
    priority_queue <P, vector<P>, greater<P> > q;
    q.push(P(d[st], st));
    while(!q.empty()) {
        int x = q.top().first, y = q.top().second;
        q.pop();
        if(d[y] < x) continue;
        for(int i = 0; i < v[y].size(); i++) {
            int t = v[y][i].t, w = v[y][i].w, c = v[y][i].c;
            if(w > 0 && d[t] > d[y] + c + h[y] - h[t]) {
                d[t] = d[y] + c + h[y] - h[t];
                pre[t] = y;
                id[t] = i;
                q.push(P(d[t], t));
            }
        }
    }
}
int ans, flow;
void mcf() {
    while(flow > 0) {
        dijkstra();
        for(int i = st; i <= ed; i++) {
            h[i] += d[i];
        }
        int f = flow;
        for(int i = ed; i != st; i = pre[i]) {
            f = min(f, v[pre[i]][id[i]].w);
        }
        flow -= f;
        ans += f * h[ed];
        for(int i = ed; i != st; i = pre[i]) {
            edge &e = v[pre[i]][id[i]];
            e.w -= f;
            v[i][e.r].w += f;
        }
    }
    // 最小费用最大流：d[i] == INF返回
}