Codeforces 1131F (并查集，union-find sets)

用并查集构造一颗树即可。
关键在于fa()中的合并路径，更新父节点，但不更新子节点。
方法1，速度较快，内存占用小，不直观:

#include <cstdio>
#include <algorithm>
#include <vector>
#include <cstring>

using namespace std;

#define maxn 150015

int n;
int num[maxn*2][3]; // fa c1 c2
int cnt;

int fa(int a){
    int b = a;
    while(num[b][0]!=b){
        b = num[b][0];
    } 
    while(num[a][0]!=b){
        int c = num[a][0];
        num[a][0] = b;
        a = c;
    }
    return b;
}

void solve(int a,int b){
    int af = fa(a);
    int bf = fa(b);
    num[af][0] = cnt;
    num[bf][0] = cnt;
    num[cnt][1] = af;
    num[cnt][2] = bf;
    cnt++;
}

void print(int a){
    int c1 = num[a][1];
    int c2 = num[a][2];
    if(c1<=n) printf("%d ",c1);
    else print(c1);
    if(c2<=n) printf("%d ",c2);
    else print(c2);
}

void tprint(){
    for(int i=1;i<10;i++){
        printf("%d %d %d %d\n",i,num[i][0],num[i][1],num[i][2]);
    }
}

int main(){
    scanf("%d",&n);
    memset(num,0,sizeof(num));
    for(int i=1;i<=2*n;i++) num[i][0] = i;
    cnt = n + 1;
    for(int i=0;i<n-1;i++){
        int a,b;
        scanf("%d%d",&a,&b);
        solve(a,b);
        //tprint();
    }
    print(cnt-1);
    printf("\n");
    return 0;
}

方法2，速度较慢，内存占用多，直观：

#include <cstdio>
#include <vector>
#include <algorithm>
#include <cstring>

using namespace std;

#define maxn 150015

int n;
int fa[maxn];
vector<int> cld[maxn];

int f(int a){
    int b = a;
    while(fa[a] != a){
        a = fa[a];
    }
    while(fa[b] != a){
        int c = fa[b];
        fa[b] = a;
        b = c;
    }
    return a;
}

void solve(int a,int b){
    int af = f(a);
    int bf = f(b);
    int cf = af;
    af = min(af,bf);
    bf = max(cf,bf);
    cld[af].insert(cld[af].end(), cld[bf].begin(), cld[bf].end());
    fa[bf] = af;
}

void print(){
    for(int i=1;i<=n;i++){
        for(vector<int>::iterator p = cld[i].begin();p!=cld[i].end();p++){
            printf("%d ",*p);
        }
        printf("\n");
    }
}

int main(){
    memset(fa,0,sizeof(fa));
    scanf("%d",&n);
    for(int i=0;i<=n;i++){
        fa[i] = i;
        cld[i].push_back(i);
    }
    for(int i=1;i<n;i++){
        int a,b,c;
        scanf("%d%d",&a,&b);
        solve(a,b);
        //print();
    }
    for(vector<int>::iterator p = cld[1].begin();p!=cld[1].end();p++){
        printf("%d ",*(p));
    }
    printf("\n");
    return 0;
}