$Tarjan$ 缩点中更新 $low$ 数组时，在 $else$ $if$ 的下面是用 $low[next]$ 还是 $dfn[next]$ ？？为什么都A了？？ $low[next]$ 等于 $dfn[next]$ ？？

for(int i = 0; i < SIZ; i++) {
	int next = v[now][i];
	if(!dfn[next]) {
		Tarjan(next);
		low[now] = min(low[now], low[next]);
	}
	else if(instack[next])
		low[now] = min(low[now], low[next]);
}

这道题的完整代码

#include <queue>
#include <stack>
#include <cstdio>
#include <vector>
#include <cstring>
#include <algorithm>
using namespace std;
const int MAXN = 1e5 + 5;
vector<int> v[MAXN], Dag[MAXN];
stack<int> s;
queue<int> q;
int dfn[MAXN], low[MAXN], belong[MAXN], elem[MAXN];
bool instack[MAXN], vis[MAXN];
int tim, cnt_set;
int w[MAXN], maxn[MAXN], minn[MAXN], in[MAXN], dp[MAXN];
int n, m, ans;
void Topo() {
	vis[belong[1]] = 1;
	for(int i = 1; i <= cnt_set; i++)
		if(!in[i])
			q.push(i);
	while(!q.empty()) {
		int now = q.front(); q.pop();
		int SIZ = Dag[now].size();
		for(int i = 0; i < SIZ; i++) {
			int next = Dag[now][i];
			if(vis[now]) {
				vis[next] = 1;
				minn[next] = min(minn[next], minn[now]);
				dp[next] = max(dp[now], max(dp[next], maxn[next] - minn[next]));
			}
			if(!(--in[next]))
				q.push(next);
		}
	}
	printf("%d", dp[belong[n]]);
}
void Tarjan(int now) {
	low[now] = dfn[now] = ++tim;
	instack[now] = true;
	s.push(now);
	int SIZ = v[now].size();
	for(int i = 0; i < SIZ; i++) {
		int next = v[now][i];
		if(!dfn[next]) {
			Tarjan(next);
			low[now] = min(low[now], low[next]);
		}
		else if(instack[next])
			low[now] = min(low[now], low[next]);
	}
	if(dfn[now] == low[now]) {
		cnt_set += 1;
		int Top = -1;
		while(!s.empty() && Top != now) {
			elem[cnt_set]++;
			Top = s.top();
			belong[Top] = cnt_set;
			instack[Top] = false;
			s.pop();
		}
	}
}
void Build() {
	for(int i = 1; i <= n; i++)
		if(!dfn[i])
			Tarjan(i);
	memset(minn, 0x7f, sizeof(minn));
	for(int i = 1; i <= n; i++) {
		maxn[belong[i]] = max(maxn[belong[i]], w[i]);
		minn[belong[i]] = min(minn[belong[i]], w[i]);
	}
	for(int i = 1; i <= n; i++) {
		int SIZ = v[i].size();
		for(int j = 0; j < SIZ; j++) {
			int next = v[i][j];
			if(belong[i] != belong[next]) {
				Dag[belong[i]].push_back(belong[next]);
				in[belong[next]]++;
			}
		}
	}
}
void Read() {
	scanf("%d %d", &n, &m);
	for(int i = 1; i <= n; i++)
		scanf("%d", &w[i]);
	for(int i = 1; i <= m; i++) {
		int U, V, F;
		scanf("%d %d %d", &U, &V, &F);
		if(F == 1)
			v[U].push_back(V);
		else {
			v[U].push_back(V);
			v[V].push_back(U);
		}
	}
}
int main() {
	Read();
	Build();
	Topo();
	return 0;
}