模板 

#include <bits/stdc++.h>
using namespace std;
const int max_n = 1e5 + 10;
int pre[max_n];
int findf(int root)
{
    if(root == pre[root])
        return root;
    else{
        root = findf(pre[root]);
        return root;
    } // 查找+压缩路径
}
void mer(int root1, int root2) //比对
{
    int t1 = findf(root1);
    int t2 = findf(root2);
    if(t1 != t2)
    {
        pre[t1] = t2;
    }
}
int main()
{
    int n;
    cin>>n;
    for(int i = 1; i <= n ;i++)
        pre[i] = i;
}

https://cn.vjudge.net/problem/POJ-2236

#include <iostream>
#include <memory.h>
#include <cstdio>
using namespace std;
#define mem(a, b) memset(a, b, sizeof(a))
int pre[1005];
struct node
{
    double x, y;
}s[1005];
bool vis[1005];
int findf(int root)
{
    if(pre[root] == -1)
        return -1;
    else if(root == pre[root])
        return root;
    else{
        root = findf(pre[root]);
        return root;
    }
}
void mer(int root1, int root2)
{
    int t1 = findf(root1);
    int t2 = findf(root2);
    if(t1 != t2)
    {
        pre[t1] = t2;
    }
}
int main()
{
    int n, d;
    scanf("%d%d", &n, &d);
    mem(vis, false);
    mem(pre, -1);
    for(int i = 1; i <= n; i++)
    {
        scanf("%lf%lf", &s[i].x, &s[i].y);
    }
    getchar();
    char jud;
    while(~scanf("%c", &jud))
    {
        if(jud == 'S'){
            int data1, data2;
            scanf("%d%d", &data1, &data2);
            getchar();
            int t1 = findf(data1);
            int t2 = findf(data2);
            if(t1 == t2)
                printf("SUCCESS\n");
            else
                printf("FAIL\n");
        }
        else{
            int data;
            scanf("%d", &data);
            getchar();
            vis[data] = true;
            pre[data] = data;
            for(int i = 1; i <= n; i++)
            {
                if(vis[i]&&i!=data){
                    double x = s[i].x-s[data].x;
                    double y = s[i].y-s[data].y;
                    if(x*x+y*y<=d*d)
                        mer(i, data);
                }
            }
        }
    }
    return 0;
}