并查集/DFS——BZOJ1529/Luogu3420 [POI2005]SKA-Piggy Banks

#include<bits/stdc++.h>
using namespace std;
int fa[1000001];
inline int getfather(int v){
if(fa[v]==v)return v;
fa[v]=getfather(fa[v]);
return fa[v];
}
int main()
{
int n,ans;scanf("%d",&n);
for(int i=1;i<=n;i++)fa[i]=i;
ans=n;//一开始假设全砸
for(int i=1;i<=n;i++){
int x;scanf("%d",&x);
int xx=getfather(i),yy=getfather(x);
if(xx==yy)continue;
ans--;//合并同一连通块少砸一个
fa[xx]=yy;
}
printf("%d",ans);
return 0;
}

#include<bits/stdc++.h>
using namespace std;
int nedge=0,p[2000001],nex[2000001],head[2000001];
bool b[1000001]={0};
inline void addedge(int a,int b){
p[++nedge]=b;nex[nedge]=head[a];
head[a]=nedge;
}
inline void dfs(int x){
b[x]=1;
for(int k=head[x];k;k=nex[k])if(!b[p[k]])dfs(p[k]);
}
int main()
{
int n,ans=0;scanf("%d",&n);
for(int i=1;i<=n;i++){
int x;scanf("%d",&x);
addedge(i,x);addedge(x,i);
}
for(int i=1;i<=n;i++)if(!b[i]){
ans++;//连通块数+1
dfs(i);
}
printf("%d",ans);
return 0;
}