UVa 11324

题目链接：https://onlinejudge.org/index.php?option=com_onlinejudge&Itemid=8&page=show_problem&category=0&problem=2299&mosmsg=Submission+received+with+ID+26582032

缩点以后求最长路，拓扑和记搜都可以

一定要注意所开数组的大小，边的数组不要开成点的，因为数组开小 WA 了一个小时，实在不行就把数组使劲往大开

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;

const int maxn = 1010;

int T, n, m;
int u[50010], v[50010], in[maxn];

vector<int> G[maxn];
int h[maxn], cnt = 0;
struct E{
	int to, next;
}e[50010];
void add(int u, int v){
	e[++cnt].to = v;
	e[cnt].next = h[u];
	h[u] = cnt;
}

int low[maxn], st[maxn], sccno[maxn], sz[maxn], dfn = 0, scc_cnt = 0;

stack<int> S;

void dfs(int u){
	low[u] = st[u] = ++dfn;
	S.push(u);
	
	for(int i = h[u] ; i != -1 ; i = e[i].next){
		int v = e[i].to;
		if(!st[v]){
			dfs(v);
			low[u] = min(low[u], low[v]);
		} else if(!sccno[v]){
			low[u] = min(low[u], st[v]);
		}
	}
	
	if(low[u] == st[u]){
		++scc_cnt;
		for(;;){
			int x = S.top(); S.pop();
			sccno[x] = scc_cnt;
			++sz[scc_cnt];
			if(x == u) break;
		}
	}
}

void tarjan(){
	memset(low, 0, sizeof(low));
	memset(st, 0, sizeof(st));
	memset(sccno, 0, sizeof(sccno));
	dfn = 0, scc_cnt = 0;
	
	for(int i = 1 ; i <= n ; ++i){
		if(!st[i]) dfs(i);
	}
}

int dp[maxn];
vector<int> path;

void topo(){
	queue<int> q;
	path.clear(); memset(dp, 0, sizeof(dp));
	
	for(int i = 1 ; i <= scc_cnt ; ++i){
		if(!in[i]){
			q.push(i);
			dp[i] = sz[i];
			path.push_back(i);
		}
	}
	
	while(!q.empty()){
		int u = q.front(); q.pop();
		for(int i = h[u] ; i != -1 ; i = e[i].next){
			int v = e[i].to;
			--in[v];
			if(!in[v]){
				q.push(v);
				path.push_back(v);
			}
		}
	}
	
	for(auto u : path){
		for(int i = h[u] ; i != -1 ; i = e[i].next){
			int v = e[i].to;
			dp[v] = max(dp[v], dp[u] + sz[v]);
		}
	}
}


int DP(int u){
	if(dp[u] >= 0) return dp[u];
	dp[u] = sz[u];
	for(int i = h[u] ; i != -1 ; i = e[i].next){
		int v = e[i].to;
		dp[u] = max(dp[u], DP(v) + sz[u]);
	}
	return dp[u];
}

ll read(){ ll s = 0, f = 1; char ch = getchar(); while(ch < '0' || ch > '9'){ if(ch == '-') f = -1; ch = getchar(); } while(ch >= '0' && ch <= '9'){ s = s * 10 + ch - '0'; ch = getchar(); } return s * f; }

int main(){
	scanf("%d", &T);
	while(T--){
		memset(in, 0, sizeof(in));
		memset(h, -1, sizeof(h)); cnt = 0;
		memset(sz, 0, sizeof(sz));
		scanf("%d%d", &n, &m);
				
		for(int i = 1 ; i <= m ; ++i){
			scanf("%d%d", &u[i], &v[i]);
			add(u[i], v[i]);
		}

    memset(sz, 0, sizeof(sz));
    tarjan();

 	memset(h, -1, sizeof(h)); cnt = 0; 
	for(int i = 1 ; i <= m ; ++i){
		if(sccno[u[i]] != sccno[v[i]]){
			add(sccno[u[i]], sccno[v[i]]);
			++in[sccno[v[i]]];
		}
	}

    int ans = 0;
    topo();
    for(int i = 1 ; i <= n ; ++i) ans = max(ans, dp[i]);
//    memset(dp, -1, sizeof(dp)); 
//    for(int i = 1; i <= scc_cnt; i++) ans = max(ans, DP(i));
 
    printf("%d
", ans);
}
	return 0;
}