#include <bits/stdc++.h>
#define inf 0x3f3f3f3f
#define N 11010
#define M 120010
using namespace std;
int n, m, s, t, sum, cc[N], use[N], yflow[N], flag[N], cntcc[N];
char xB[1 << 15], *xS = xB, *xT = xB;
#define getchar() (xS == xT && (xT = (xS = xB) + fread(xB, 1, 1 << 15, stdin), xS == xT) ? 0 : *xS++)
inline int read() {
	int x = 0, f = 1;
	char ch = getchar();
	while (ch < '0' || ch > '9') {
		if (ch == '-')
			f = -1;
		ch = getchar();
	}
	while (ch >= '0' && ch <= '9') {
		x = x * 10 + ch - '0';
		ch = getchar();
	}
	return x * f;
}
struct cmp {
	bool operator()(int a, int b) const {
		return cc[a] < cc[b];
	}
};
priority_queue<int, vector<int>, cmp> q;
struct node {
	int to, next, val;
} e[M << 1];
int head[N], last[N];
void add(int x, int y, int val, int id) {
	if (head[x] == 0) head[x] = id;
	else e[last[x]].next = id;
	last[x] = id, e[id].val = val, e[id].to = y;
}
void bfs() {
	memset(cc, 0x3f, sizeof(cc));
	use[1] = t, cc[t] = 0;
	int u = 0, v = 1;
	while (u < v) {
		++u;
		int fst = use[u];
		for (int i = head[fst]; i != 0; i = e[i].next) {
			if (cc[e[i].to] != inf)
				continue;
			if (e[i ^ 1].val == 0)
				continue;
			++v, cc[e[i].to] = cc[fst] + 1, use[v] = e[i].to;
		}
	}
	use[s] = n;
}
void HLPP() {
	bfs();
	if (cc[s] == inf) return;
	cc[s] = n;
	for (int i = 1; i <= n; i++) if (cc[i] != inf) cntcc[cc[i]]++;
	for (int i = head[s]; i != 0; i = e[i].next) {
		yflow[e[i].to] = e[i].val, e[i ^ 1].val += e[i].val, e[i].val = 0;
		if (e[i].to != s && e[i].to != t && flag[e[i].to] == 0)
			flag[e[i].to] = 1, q.push(e[i].to);
	}
	while (!q.empty()) {
		int now = q.top();
		q.pop();
		flag[now] = 0;
		if (cc[now] == n + 1) continue;
		for (int i = head[now]; i != 0; i = e[i].next)
			if (cc[now] == cc[e[i].to] + 1) {
				int flow = min(yflow[now], e[i].val);
				if (flow == 0)
					continue;
				e[i].val -= flow, e[i ^ 1].val += flow;
				yflow[now] -= flow, yflow[e[i].to] += flow;
				if (e[i].to != s && e[i].to != t && flag[e[i].to] == 0) flag[e[i].to] = 1, q.push(e[i].to);
				if (!yflow[now]) break;
			}
		if (yflow[now]) {
			--cntcc[cc[now]];
			if (cntcc[cc[now]] == 0) {
				for (int i = 1; i <= n; i++)
					if (cc[i] > cc[now] && i != s && i != t && cc[now] <= now)
						cc[now] = n + 1, yflow[i] = 0;
				continue;
			}
			int tmp = m + 1;
			for (int i = head[now]; i != 0; i = e[i].next) if (e[i].val) tmp = min(cc[e[i].to] + 1, tmp);
			cc[now] = tmp;
			++cntcc[cc[now]];
			flag[now] = 1;
			q.push(now);
		}
	}
}
signed main() {
	int x, y, z;
	n = read(), m = read(), s = read(), t = read();
	for (int i = 1; i <= m; i++) {
		x = read(), y = read(), z = read();
		add(x, y, z, i * 2);
		add(y, x, 0, i * 2 + 1);
	}
	HLPP();
	printf("%d\n", yflow[t]);
	return 0;
}

https://www.luogu.com.cn/record/33995567

https://loj.ac/submission/820057

这还是我请了金钩老师改的代码QAQ