注意第 39 行加了 "NEEDFIX" 注释的无用点优化。

如果 dfs 时向一个点推流却发现返回的流量是 $0$，那说明那个点无剩余流量（增广完毕），应该被舍弃。但加上那一行会导致 TLE，哪里写假了？

#include <iostream>
#include <cstring>
#include <queue>
using namespace std;
/* d0j1a_1701 FastIO ver. 5.0 */
/* 省略快读代码 */
int beg[210], nxt[10010], to[10010], cur[210], tot = -1;
int n, m, s, t, id[210][210], dep[210];
long long val[10010];
inline void add(int u, int v, long long w) {
	nxt[++tot] = beg[u], beg[u] = tot, val[tot] = w, to[tot] = v;
	nxt[++tot] = beg[v], beg[v] = tot, val[tot] = 0, to[tot] = u;
}
inline bool bfs() {
	queue<int> q;
	memset(dep, -1, sizeof(dep));
	q.emplace(s), dep[s] = 0, cur[s] = beg[s];
	while(q.size()) {
		int u = q.front();
		q.pop();
		for(int i = beg[u]; ~i; i = nxt[i])
			if(val[i] && dep[to[i]] < 0) {
				int v = to[i];
				dep[v] = dep[u] + 1;
				cur[v] = beg[v];
				q.emplace(v);
				if(v == t)	return 1;
			}
	}
	return 0;
}
long long dfs(int u, long long rest = 1e18) {
	if(u == t)	return rest;
	long long flow = 0;
	for(int i = cur[u]; ~i; i = nxt[i])
		if(dep[to[i]] == dep[u] + 1) {
			int v = to[i];
			auto f = dfs(v, min(rest, val[i]));
			//if(!f)	dep[v] = -1; // NEEDFIX: 无用点优化
			val[i] -= f, val[i ^ 1] += f;
			flow += f, rest -= f;
			if(rest <= 0)	return flow; // 当前弧优化（推送量用尽）
		}
	return flow;
}
int main() {
	memset(beg, -1, sizeof(beg));
	io.read(n, m, s, t);
	for(int i = 1, u, v, w; i <= m; i++) {
		io.read(u, v, w);
		if(id[u][v])	val[id[u][v]] += w;
		else add(u, v, w), id[u][v] = tot - 1;
	}
	long long res = 0;
	while(bfs())	res += dfs(s);
	io.writeln(res);
	//system("pause");
	return 0;
}