loj #127. 最大流 加强版

// luogu-judger-enable-o2
#include <cstdio>
#include <cstdlib>
#include <iostream>
#include <cstring>
#include <algorithm>

using namespace std;

const int N = 10010, M = 250010;
const unsigned long long INF = (1 << 31) - 1;

int n, m, S, T;
int h[N], e[M], f[M], ne[M], idx;
int q[N], d[N], nh[N];

inline void add(int a, int b, int c) { e[idx] = b, f[idx] = c, ne[idx] = h[a], h[a] = idx++; }

bool bfs() {
    memset(d, -1, sizeof d);
    q[0] = S, d[S] = 0, nh[S] = h[S];
    int hh = 0, tt = 1;
    int flag = 0;
    while (hh < tt) {
        int t = q[hh++];
        for (int i = h[t]; ~i; i = ne[i]) {
            int v = e[i];
            if (d[v] == -1 && f[i]) {
                d[v] = d[t] + 1;
                nh[v] = h[v];  // 可以在开头全copy
                if (v == T)
                    flag = 1;  // return true;
                q[tt++] = v;
            }
        }
    }
    return flag;
}

int find(int u, int limit) {
    if (u == T)
        return limit;
    int flow = 0;

    for (int i = nh[u]; ~i && flow < limit; i = ne[i]) {  // 1. nh 优化， 2. 增广最大量优化
        nh[u] = i;                                        // nh当前可行弧优化
        int v = e[i];
        if (d[v] == d[u] + 1 && f[i]) {
            int t = find(v, min(f[i], limit - flow));
            if (t)
                flow += t, f[i] -= t;  //, f[i ^ 1] += t;
            else
                d[v] = -1;  // 3. 废点优化
        }
    }
    return flow;
}

int dinic() {
    int res = 0, flow;
    while (bfs())
        while (flow = find(S, INF)) res += flow;
    return res;
}

int main() {
    scanf("%d%d%d%d", &n, &m, &S, &T);
    memset(h, -1, sizeof h);
    while (m--) {
        int a, b, c;
        scanf("%d%d%d", &a, &b, &c);
        add(a, b, c), add(b, a, 0);
    }
    printf("%d\n", dinic());
    return 0;
}