邻接表存图，输出连通分量里顶点的下标：

#include <bits/stdc++.h>
using namespace std;
struct Edge{
    int to;
    Edge* next;
};
struct Vex{
    int id;
    Edge* first; 
}vex[100];
struct Tree{
    int id;
    Tree *child,*sibling;
};
void AddEdge(int u,int v){
    if(!vex[u].first){
        vex[u].first=new Edge;
        vex[u].first->to=v;
        vex[u].first->next=NULL;
        return;
    }
    Edge *p=new Edge;
    p->next=vex[u].first;
    p->to=v;
    vex[u].first=p;
}
int n;
void CreatGraph(){
    int u,v;
    cin>>n;
    for(int i=0;i<n;i++) vex[i].id=i;
    while(cin>>u>>v){
        AddEdge(u,v);
        AddEdge(v,u);
    }
}
bool vis[100];
void DFSTree(int v){
    for(Edge *e=vex[v].first;e;e=e->next){
        int w=e->to;
        if(!vis[w]){
            cout<<w<<' ';
            vis[w]=1;
            DFSTree(w);
        }
    }
}
int dfs(){
    int res=0;
    for(int i=0;i<n;i++) vis[i]=0;
    for(int v=0;v<n;v++){
        if(!vis[v]){
            vis[v]=1;
            res++;
            cout<<v<<' ';
            DFSTree(v);
            cout<<endl;
        }
    }
    return res;
}
int main(){
    freopen("in.txt","r",stdin);
    freopen("out.txt","w",stdout);
    CreatGraph();
    cout<<dfs();   
    return 0;
}