【BZOJ】1529 [POI2005]ska Piggy banks

#include<cstdio>
#include<algorithm>
using namespace std;
const int maxn=1000010;
struct edge{int u,v,from;}e[maxn];
int n,tot,top,mark,color,col[maxn],dfn[maxn],low[maxn],s[maxn],lack[maxn],in[maxn],first[maxn];
void insert(int u,int v)
{tot++;e[tot].u=u;e[tot].v=v;e[tot].from=first[u];first[u]=tot;}
void tarjan(int x)
{
    dfn[x]=low[x]=++mark;
    s[++top]=x;lack[x]=top;
    for(int i=first[x];i;i=e[i].from)
     {
         int y=e[i].v;
         if(!dfn[y])
          {
              tarjan(y);
              low[x]=min(low[x],low[y]);
          }
         else if(!col[y])low[x]=min(low[x],dfn[y]);
     }
    if(dfn[x]==low[x])
     {
         color++;
         for(int i=lack[x];i<=top;i++)col[s[i]]=color;
         top=lack[x]-1;
     }
}
int main()
{
    scanf("%d",&n);
    for(int i=1;i<=n;i++)
     {
         int u;
         scanf("%d",&u);
         insert(u,i);
     }
    for(int i=1;i<=n;i++)if(!dfn[i])tarjan(i);
    for(int i=1;i<=tot;i++)
     if(col[e[i].u]!=col[e[i].v])in[col[e[i].v]]++;
    int ans=0;
    for(int i=1;i<=color;i++)if(in[i]==0)ans++;
    printf("%d",ans);
    return 0;
}

#include<cstdio>
#include<algorithm>
using namespace std;
const int maxn=1000010;
int f[maxn],n;
int getfa(int x)
{return f[x]==x?x:f[x]=getfa(f[x]);}
int main()
{
    scanf("%d",&n);
    for(int i=1;i<=n;i++)f[i]=i;
    for(int i=1;i<=n;i++)
     {
         int u;
        scanf("%d",&u);
         if(getfa(i)!=getfa(u))f[f[i]]=f[u];
     }
    int ans=0;
    for(int i=1;i<=n;i++)if(f[i]==i)ans++;
    printf("%d",ans);
    return 0;
}