网络流--最大流--hlpp（预流推进）模板

//500ms 秒掉洛谷推流问题
#include <algorithm>
#include <iostream>
#include <cstring>
#include <vector>
#include <queue>
using namespace std;
typedef long long LL;
typedef long long F_type;
const int MAXN = 1.2e3 + 10, INF = 0x3f3f3f3f;
const LL LINF = (LL)INF << 32 | INF;
struct Edge
{
    int v, rev;
    F_type cap;
    Edge(int a, F_type b, int c) : v(a), rev(c), cap(b) {}
};
const F_type maxf=LINF;
F_type exflow[MAXN];
int h[MAXN], cnt[MAXN];
int ht, N, S, T, labelcnt;
vector<Edge> G[MAXN];
vector<int> hq[MAXN];
void clear(int n = MAXN - 1)
{
    ht = labelcnt = 0;
    for (int i = 0; i <= n; i++)
        G[i].clear();
}
void addEdge(int u, int v, F_type cap)
{
    G[u].emplace_back(v, cap, G[v].size());
    G[v].emplace_back(u, 0, G[u].size() - 1);
}
void update(int u, int newh)
{
    ++labelcnt;
    if (h[u] != N + 1)
        --cnt[h[u]];
    h[u] = newh;
    if (newh == N + 1)
        return;
    ++cnt[ht = newh];
    if (exflow[u] > 0)
        hq[newh].push_back(u);
}
void globalRelabel()
{
    queue<int> q;
    for (int i = 0; i <= N + 1; i++)
        hq[i].clear();
    for (int i = 0; i <= N; i++)
        h[i] = N + 1, cnt[i] = 0;
    q.push(T);
    labelcnt = ht = h[T] = 0;
    while (!q.empty())
    {
        int u = q.front();
        q.pop();
        for (Edge& e : G[u])
        {
            if (h[e.v] == N + 1 && G[e.v][e.rev].cap)
            {
                update(e.v, h[u] + 1);
                q.push(e.v);
            }
        }
        ht = h[u];
    }
}
void push(int u, Edge& e)
{
    if (exflow[e.v] == 0)
        hq[h[e.v]].push_back(e.v);
    F_type df = min(exflow[u], e.cap);
    e.cap -= df;
    G[e.v][e.rev].cap += df;
    exflow[u] -= df;
    exflow[e.v] += df;
}
void discharge(int u)
{
    int nxth = N + 1;
    for (Edge& e : G[u])
        if (e.cap)
        {
            if (h[u] == h[e.v] + 1)
            {
                push(u, e);
                if (exflow[u] <= 0)
                    return;
            }
            else
                nxth = min(nxth, h[e.v] + 1);
        }
    if (cnt[h[u]] > 1)
        update(u, nxth);
    else
        for (; ht >= h[u]; hq[ht--].clear())
        {
            for (int& j : hq[ht])
                update(j, N + 1);
        }
}
F_type maxFlow(int s, int t, int n)
{
    S = s, T = t, N = n;
    memset(exflow, 0, sizeof(exflow));
    exflow[S] = maxf;
    exflow[T] = -maxf;
    globalRelabel();
    for (Edge& e : G[S])
        push(S, e);
    for (; ht >= 0; --ht)
    {
        while (!hq[ht].empty())
        {
            int u = hq[ht].back();
            hq[ht].pop_back();
            discharge(u);
            if (labelcnt > (N << 2))
                globalRelabel();
        }
    }
    return exflow[T] + maxf;
}

int main()
{
    int n, m, s, t, u, v, w;
    scanf("%d%d%d%d", &n, &m, &s, &t);
    while (m--)
    {
        scanf("%d%d%d", &u, &v, &w);
        addEdge(u, v, w);
    }
    printf("%d", maxFlow(s, t, n));
    return 0;
}