uva11324(tarjan缩点 ＋ DAG图上的简单dp)

思路：先缩点，再构图，最后dp。dp[u] = max(dp[u],dfs(v) + val[u]);

#include <iostream>
#include <stack>
#include <vector>
#include <cstdio>
#include <cstring>
using namespace std;
const int maxn = 1e3 + 10;
int low[maxn],pre[maxn],sccno[maxn];
int scc_cnt,dfs_colok;
stack<int> s;
vector<int> G[maxn],mp[maxn];
void dfs(int u){
	low[u] = pre[u] = ++dfs_colok;
	s.push(u);
	for (int i = 0;i < G[u].size();++i){
		int v = G[u][i];
		if (!pre[v]){
			dfs(v);
			low[u] = min(low[v],low[u]);
		}else if (!sccno[v]) low[u] = min(low[u],pre[v]);
	}
	if (low[u] == pre[u]){
		scc_cnt++;
		for (;;){
			int x = s.top();
			s.pop();
			sccno[x] = scc_cnt;
			if (x == u) break;
		}
	}
}
void find_scc(int n){
	dfs_colok = scc_cnt = 0;
	memset(sccno, 0, sizeof sccno);
	memset(pre, 0,sizeof pre);
	for (int i = 0;i < n;i++)
		if (!pre[i]) dfs(i);
}
int val[maxn];
void build(int n){
	for (int i = 1;i <= scc_cnt;i++)
		mp[i].clear();
	memset(val, 0,sizeof val);
	for (int i = 0;i < n;i++){
		val[sccno[i]]++;
		// cout << val[sccno[i]] << endl;
	}
	for (int i = 0;i < n;i++){
		for (int j = 0;j < G[i].size();j++){
			int v = G[i][j];
			if (sccno[i] != sccno[v]){
				mp[sccno[i]].push_back(sccno[v]);
			}
		}
	}
}
int dp[maxn];
int search(int u){
	if (dp[u] != -1) return dp[u];
	dp[u] = val[u];
	for (int i = 0;i < mp[u].size();i++){
		int v = mp[u][i];
		dp[u] = max(dp[u],search(v) + val[u]);
	}
	// cout << dp[u] << endl;
	return dp[u];
}
int main(){
	// freopen("in.txt","r",stdin);
	// freopen("out","w",stdout);
	int n,m,t;
	scanf("%d",&t);
	while(t--){
		scanf("%d%d",&n,&m);
		for (int i = 0;i < n;i++)
			G[i].clear();
		int a,b;
		for (int i = 1;i <= m;i++){
			scanf("%d%d",&a,&b);
			a--;
			b--;
			G[a].push_back(b);
		}
		find_scc(n);
		// cout << "scc_cnt = " << scc_cnt << endl;
		build(n);
		memset(dp, -1,sizeof dp);
		int ans = 0;
		// for (int i = 1;i <= scc_cnt;i++)
		for (int i = 0;i < n;i++)
			ans = max(ans,search(sccno[i]));
		printf("%d\n", ans);
	}
	return 0;
}