Go: package main

import (
	"fmt"
	"bufio"
	"os"
	"strconv"
)

func main() {
	var n,m int
	fmt.Scan(&n)
	group := make([]int, n)
	g := make([][]int, n)
	for i := 0; i < n; i++ {
		g[i] = []int{}
		group[i] = -1
	}
	
	fmt.Scan(&m)
	sc := bufio.NewScanner(os.Stdin)
	sc.Split(bufio.ScanWords)
	for i := 0; i < m; i++ {
		sc.Scan()
		s, _ := strconv.Atoi(sc.Text())
		sc.Scan()
		t, _ := strconv.Atoi(sc.Text())
		
		g[s] = append(g[s], t)
		g[t] = append(g[t], s)
	}
	for i := 0; i < n; i++ {
		if group[i] != -1 {
			continue
		}
		q := []int{i}
		for len(q) > 0 {
			pop := q[0]
			if len(q) > 1 {
				q = q[1:]
			} else {
				q = []int{}
			}
			if group[pop] == -1 {
				q = append(q, g[pop]...)
				group[pop] = i
			}
		}
	}
	sc.Scan()
	q, _ := strconv.Atoi(sc.Text())
	for i := 0; i < q; i++ {
		sc.Scan()
		s, _ := strconv.Atoi(sc.Text())
		sc.Scan()
		t, _ := strconv.Atoi(sc.Text())

		if group[s] == group[t] {
			fmt.Println("yes")
		} else {
			fmt.Println("no")
		}
	}
}

func root(i int, group *[]int) int {
	if i == (*group)[i] {
		return i
	}
	(*group)[i] = root((*group)[i], group)
	return (*group)[i]
}

C++: #ifndef ___Class_Union_Find
#define ___Class_Union_Find
 
#include <vector>
#include <utility>
#include <algorithm>
 
using namespace std;
 
class Union_Find
{
private:
 
    vector<int> data;
 
public:
 
    Union_Find(unsigned size_) : data(size_, -1) { }
 
    void unite(int x, int y);
     
    bool same(int x, int y);
 
    int root(int x);
};
 
void Union_Find::unite(int x, int y)
{
    x = root(x);
    y = root(y);
 
    if (x != y)
    {
        if (data[x] < data[y])
        {
            swap(x, y);
        }
 
        data[x] += data[y];
        data[y] = x;
    }
}
 
bool Union_Find::same(int x, int y)
{
    return root(x) == root(y);
}
 
int Union_Find::root(int x)
{
    return data[x] < 0 ? x : data[x] = root(data[x]);
}
 
#endif // ___Class_Union_Find
 
 
// ------ main ------ //
 
#include <cstdio>
 
using namespace std;
 
int n, m, q, x, y;
 
int main()
{
    scanf("%d", &n);
    scanf("%d", &m);
 
    Union_Find u(n);
 
    for (int i = 0; i < m; i++)
    {
        scanf("%d", &x);
        scanf("%d", &y);
 
        u.unite(x, y);
    }
    
    scanf("%d", &q);
    
    for(int i = 0; i < q; i++)
    {
        scanf("%d", &x);
        scanf("%d", &y);
        
        printf("%s\n", u.same(x, y) ? "yes" : "no");
    }
 
    return 0;
}

C++: #include <iostream>
#include <stack>
using namespace std;

const long N = 10000;
const long WHITE = 0;
const long GRAY = 1;
const long BLACK = 2;
const long NIL = -1;

long n, M[N][N];
long color[N], cc[N], cp = 0;
long nt[N];

long next(long u){
	for (long v = nt[u]; v <= n; v++){
		nt[u] = v + 1;
		if (M[u][v]) return v;
	}
	return NIL;
}

void dfs_visit(long r){
	for (long i = 0; i < n; i++) nt[i] = 0;

	stack<long> S;
	S.push(r);
	color[r] = GRAY;
	cc[r] = cp;

	while (!S.empty()){
		long u = S.top();
		long v = next(u);
		if (v != NIL){
			if (color[v] == WHITE){
				color[v] = GRAY;
				cc[v] = cp;
				S.push(v);
			}
		}
		else {
			S.pop();
			color[u] = BLACK;
		}
	}
	cp++;
}

void dfs(){
	for (long i = 0; i < n; i++){
		color[i] = WHITE;
		nt[i] = 1;
	}

	for (long u = 0; u < n; u++){
		if (color[u] == WHITE) dfs_visit(u);
	}

}

int main(){
	long m, q;
	cin >> n >> m;
	for (long i = 0; i < n; i++){
		for (long j = 0; j < n; j++) M[i][j] = 0;
	}
	for (long i = 0; i < m; i++){
		long s, t;
		cin >> s >> t;
		M[s][t] = M[t][s] = 1;
	}

	dfs();

	cin >> q;
	for (long i = 0; i < q; i++){
		long s, t;
		cin >> s >> t;
		if (cc[s] == cc[t]) cout << "yes" << endl;
		else cout << "no" << endl;
	}

	return 0;
}

C++: #include<iostream>    // ???????????????????§?????????£????????????
using namespace std;
#include<stdio.h>

typedef struct fox { int val; fox *prev, *next; } fox_t;

fox_t* init()
{
	fox_t* head = new fox_t;
	head->val = -1;
	head->next = NULL;
	head->prev = NULL;
	return head;
}

fox_t* make_tail(fox_t* head)
{
	fox_t* tail = new fox_t;
	tail->val = -1;
	head->next = tail;
	tail->next = NULL;
	tail->prev = head;
	return tail;
}

bool isEmpty(fox_t* head, fox_t* tail)
{
	return (head->next == tail ? true : false);
}

fox_t* b_insert(fox_t* tail, int data)
{
	fox_t* fox = new fox_t;
	fox->val = data;
	fox->prev = tail->prev;
	fox->next = tail;
	tail->prev->next = fox;
	tail->prev = fox;
	return fox;
}

void f_show(fox_t* head, fox_t* tail)
{
	cout << "----??????????????¨???----" << endl;
	fox_t* fox = head->next;

	while(fox != tail){
		cout << fox->val;
		fox = fox->next;
		if(fox != tail) cout << " ";
	};
	cout << endl;
	return;
}
// docking.
void unionfox(fox_t* head1, fox_t* tail1, fox_t* head2, fox_t* tail2)
{
	if(isEmpty(head1, tail1) || isEmpty(head2, tail2)) return;
	fox_t* fox = tail1->prev;
	cout << tail1->prev << endl;
	cout << head2->next << endl;
	fox->next = head2->next;
	head2->next->prev = fox;
	tail2->prev->next = tail1;
	tail1->prev = tail2->prev;
	delete tail2;
	delete head2;
}

int main()
{
	int color[100001];
	int c_num = 0;
	fox_t* foxhead[100001];
	fox_t* foxtail[100001];
	int i, n, m, q;
	int a, b;
	cin >> n >> m;
	for(i = 0; i < n; i++){
		color[i] = -1;
		foxhead[i] = init();
		foxtail[i] = make_tail(foxhead[i]);
	}
	int c_a, c_b, val;

	for(i = 0; i < m; i++){
		cin >> a >> b;
		c_a = color[a];
		c_b = color[b];
		if(c_a < c_b) swap(a, b);  // color[a] >= color[b].
		if(c_a < 0 && c_b < 0){
			color[a] = c_num; color[b] = c_num;
			b_insert(foxtail[c_num], a);
			b_insert(foxtail[c_num], b);
			c_num++;
		}else if(c_b < 0){
			color[b] = c_a;
			b_insert(foxtail[c_a], b);
		}else if(c_a == c_b){
			continue;
		}else{ // color[a] != color[b].
			// b????????¬?????????color???a???color??????????????§???union?????????
			fox_t* fox = foxhead[c_b]->next;
			while(fox != foxtail[c_b]){
				val = fox->val;
				color[val] = c_a;
				fox = fox->next;
			};
			unionfox(foxhead[c_a], foxtail[c_a], foxhead[c_b], foxtail[c_b]);
		}
	}
	cin >> q;
	for(i = 0; i < q; i++){
		cin >> a >> b;
		if(color[a] >= 0 && (color[a] == color[b])){
			printf("yes\n"); }else{ printf("no\n"); 
			}
	}
	return 0;
}

C++: #include<iostream>
#include<vector>
using namespace std;

void set_color(const vector<vector<int> >& u, int p,
               vector<int> color, int c) {
    if (color[p] != -1) return;
    color[p] = c;
    for (int i = 0; i < u[p].size(); i++) {
        set_color(u, u[p][i], color, c);
    }
}

bool is_friend(const vector<vector<int> >& u, int s, int t){
    vector<int> color(u.size(), -1);
    int c = 0;
    set_color(u, s, color, c++);
    return color[s] == color[t];
}
int main(){
    int n, m;
    cin >> n >> m;
    vector<vector<int> > u(n);
    for (int i = 0; i < m; i++) {
        int s, t;
        cin >> s >> t;
        u[s].push_back(t);
        u[t].push_back(s);
    }
    int q;
    cin >> q;
    for (int i = 0; i < q; i++) {
        int s, t;
        cin >> s >> t;
        if (is_friend(u, s, t)) {
            cout << "yes" << endl;
        } else {
            cout << "no"  << endl;
        }
    }
    return 0;
}