Codeforces Round #286 (Div. 1) B. Mr. Kitayuta's Technology （强连通分量）

#include <iostream>
#include <string.h>
#include <math.h>
#include <queue>
#include <algorithm>
#include <stdlib.h>
#include <map>
#include <set>
#include <stdio.h>
#include <time.h>
using namespace std;
#define LL long long
#define pi acos(-1.0)
#pragma comment(linker, "/STACK:1024000000")
const int mod=1e9+7;
const int INF=0x3f3f3f3f;
const double eqs=1e-9;
const int MAXN=100000+10;
int head[MAXN], cnt;
int dfn[MAXN], low[MAXN], scc, belong[MAXN], instk[MAXN], stk[MAXN], indx, tot[MAXN], top;
int vis[MAXN];
int flag, num;
struct node
{
int u, v, next;
}edge[MAXN<<1];
void add(int u, int v)
{
edge[cnt].u=u;
edge[cnt].v=v;
edge[cnt].next=head[u];
head[u]=cnt++;
}
void init()
{
memset(head,-1,sizeof(head));
cnt=indx=top=0;
memset(dfn,0,sizeof(dfn));
memset(instk,0,sizeof(instk));
memset(tot,0,sizeof(tot));
}
void tarjan(int u)
{
dfn[u]=low[u]=++indx;
stk[++top]=u;
instk[u]=1;
for(int i=head[u];i!=-1;i=edge[i].next){
int v=edge[i].v;
if(!dfn[v]){
tarjan(v);
low[u]=min(low[u],low[v]);
}
else if(instk[v]) low[u]=min(low[u],dfn[v]);
}
if(dfn[u]==low[u]){
scc++;
while(1){
int v=stk[top--];
belong[v]=scc;
instk[v]=0;
tot[scc]++;
if(u==v) break;
}
}
}
void dfs(int u)
{
num++;
vis[u]=1;
if(tot[belong[u]]>=2) flag=1;
for(int i=head[u];i!=-1;i=edge[i].next){
int v=edge[i].v;
if(vis[v]) continue ;
dfs(v);
}
}
int main()
{
int n, m, u, v, i, j, ans;
while(scanf("%d%d",&n,&m)!=EOF){
init();
while(m--){
scanf("%d%d",&u,&v);
add(u,v);
}
ans=0;
for(i=1;i<=n;i++){
if(!dfn[i])
tarjan(i);
}
m=cnt;
for(i=0;i<m;i++){
u=edge[i].u;
v=edge[i].v;
add(v,u);
}
memset(vis,0,sizeof(vis));
ans=0;
for(i=1;i<=n;i++){
if(!vis[i]){
num=flag=0;
dfs(i);
ans+=num-1+flag;
}
}
printf("%d\n",ans);
}
return 0;
}