Go: package main

import (
	"fmt"
)

var n, m int
var g [][]int
var color []int

func main() {
	fmt.Scan(&n, &m)
	g = make([][]int, n) // SNSの関係を表すグラフ
	for i := 0; i < m; i++ {
		var a, b int
		fmt.Scan(&a, &b)
		g[a] = append(g[a], b) // 無向重みなしグラフ
		g[b] = append(g[b], a)
	}
	color = make([]int, n) // 各ノードにつける色
	for i := 0; i < len(color); i++ {
		color[i] = -1 // 初期化しておく
	}
	type query struct {
		s, t int
	}
	var Q int
	fmt.Scan(&Q)
	qs := make([]query, Q)
	for i := 0; i < Q; i++ {
		fmt.Scan(&qs[i].s, &qs[i].t)
	}
	// fmt.Println("n", n, "m", m, "g", g, "Q", Q, "qs", qs)
	// 全ての頂点からdfsする
	id := 1
	for u := 0; u < n; u++ {
		if color[u] == -1 {
			dfs(u, id)
			id++
		}
	}
	// fmt.Println("color", color)
	for i := 0; i < Q; i++ {
		if color[qs[i].s] == color[qs[i].t] {
			fmt.Println("yes")
		} else {
			fmt.Println("no")
		}
	}
}

func dfs(v, c int) {
	// fmt.Printf("dfs(v %d, c %d) en:\n", v, c)
	color[v] = c
	for _, vv := range g[v] {
		// fmt.Printf("  v %d, g[v] %v, vv %d\n", v, g[v], vv)
		if color[vv] == -1 {
			dfs(vv, c)
		}
	}
}

C++: #include <stdio.h>

#define MAX_USER (100000)

/*

1)initial state

	0    1    2    3    4    5...

2)create friendship 
	friend: 0 and 1
	friend: 2 and 3

	0    2    4
    |    |
	1    3

3)and more
	friend: 2 and 4

	0       2      5
	|      / \
	1     3---4

*/



struct UserInfo
{
	long uid;
	UserInfo* next;
	UserInfo* prev;
};

int examine_friendship( const UserInfo* user_info , long uid1 , long uid2 )
{
	UserInfo* uid_ptr = user_info[ uid1 ].next;

	while( uid_ptr->uid != uid1 )
	{
		if( uid_ptr->uid == uid2 )
		{
			return 1;
		}
		uid_ptr = uid_ptr->next;
	}
	return 0;
}

int main( int argc , char** argv )
{
	//read setting
	long user_count;
	long friendship_count;
	scanf( "%ld %ld" , &user_count , &friendship_count );

	//initialize
	UserInfo*  user_info = new UserInfo[ user_count ];
	for( long i = 0 ; i < user_count ; i++ )
	{
		user_info[ i ].uid = i;
		user_info[ i ].next = &user_info[ i ];
		user_info[ i ].prev = &user_info[ i ];
	}

	//read user friendship
	for( long i = 0 ; i < friendship_count ; i++ )
	{
		long uid1;
		long uid2;
		scanf( "%ld %ld" , &uid1 , &uid2 );

		if( !examine_friendship( user_info , uid1 , uid2 ) )
		{
			//update internal data
			UserInfo* uid1_ptr = &user_info[ uid1 ];
			UserInfo* uid2_ptr = &user_info[ uid2 ];
			UserInfo* uid1_prev_ptr = uid1_ptr->prev;
			UserInfo* uid2_next_ptr = uid2_ptr->next;

			uid1_ptr->prev = uid2_ptr;
			uid2_ptr->next = uid1_ptr;

			uid1_prev_ptr->next = uid2_next_ptr;
			uid2_next_ptr->prev = uid1_prev_ptr;
		}
	}

	//read number of question
	long question_count;
	scanf( "%ld" , &question_count );

	//process question
	for( long i = 0 ; i < question_count ; i++ )
	{
		long uid1;
		long uid2;
		scanf( "%ld %ld" , &uid1 , &uid2 );

		if( examine_friendship( user_info , uid1 ,uid2 ) )
		{
			printf( "yes\n" );
		}
		else
		{
			printf( "no\n" );
		}
	}

	delete[] user_info;
	return 0;
}

C++: #include<iostream>
using namespace std;
#include<stdio.h>

typedef struct box { int val; box *prev, *next; } box_t;

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

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

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

box_t* f_insert(box_t* head, int data)
{
    box_t* box = new box_t;
    box->val = data;
    box->next = head->next;
    box->prev = head;
    head->next->prev = box;
    head->next = box;
    return box;
}

int b_pop(box_t* tail)
{
    if(!(tail->prev->prev)) return -1;
    box_t* box = tail->prev;
    int val;
    val = tail->prev->val;
    tail->prev = box->prev;
    box->prev->next = tail;
    delete box;
    return val;
}

void clear(box_t* head, box_t* tail)
{
    box_t* box1 = head->next;
    box_t* box2;
    if(box1 == tail) return;
    while(box1 != tail){
        box2 = box1->next;
        delete box1;
        box1 = box2;
    };
    head->next = tail;
    tail->prev = head;
    return;
}

int main()
{
    int G[100000][100000];
    int dist[100000];

    box_t* head = init();
    box_t* tail = make_tail(head);

    int n, i, j, e, q, v1, v2;
    scanf("%d", &n);
    for(i = 0; i < n; i++){
        for(j = 0; j < n; j++){ G[i][j] = 0; } }
    int m;
    scanf("%d", &m);
    for(int k = 0; k < m; k++){
        scanf("%d %d", &i, &j);
        G[i][0]++; G[j][0]++;
        G[i][G[i][0]] = j;
        G[j][G[j][0]] = i;
    }
    scanf("%d", &q);
    int self, friends;
    bool is_connected;

    for(int k = 0; k < q; k++){
        for(j = 0; j < n; j++) dist[j] = -1;

        scanf("%d %d", &self, &friends);
        f_insert(head, self);
        dist[self] = 0;
        is_connected = false;

        while(1){
            v1 = b_pop(tail);
            e = G[v1][0];
            for(i = 1; i <= e; i++){
                v2 = G[v1][i];
                if(dist[v2] < 0){
                    dist[v2] = dist[v1] + 1;
                    f_insert(head, v2);
                }
            }
            if(dist[friends] > 0){
                is_connected = true;
                break;
            }
            if(isEmpty(head, tail)) break;
        };
        if(is_connected){ printf("yes\n"); }else{ printf("no\n"); }
        if(!isEmpty(head, tail)) clear(head, tail);
    }

    return 0;
}

C++: #include <iostream>
#include <vector>
#include <stack>

#define inf 1000000007

using namespace std;

vector<vector<int> > connect;
vector<int> visited;
int n;

void dfs(int start,int c){
	stack<int> st;
	st.push(start);
	while(!st.empty()){
		int k = st.top();
		visited[k] = c;
		st.pop();
		for(int i=0;i<connect[k].size();++i){
			int tmp = connect[k][i];
			if(visited[tmp]==-1){
				st.push(tmp);
			}
		}
	}
}

int main(){
	int m,q;
	cin >> n >> m;
	connect.resize(n);
	visited.resize(n);
	for(int i=0;i<m;++i){
		int s,t;
		cin >> s >> t;
		connect[s].push_back(t);
		connect[t].push_back(s);
	}
	int c = 0;
	for(int i=0;i<n;++i)visited[i] = -1;
	for(int i=0;i<n;++i){
		if(visited[i] == -1){
			dfs(i,c);
			c++;
		}
	}
	cin >> q;
	vector<bool> res(q);
	for(int i=0;i<q;++i){
		int from,to;
		cin >> from >> to;
		if(visited[from] == visited[to]){
			res[i] = true;
		}else{
			res[i] = false;
		}
	}
	for(int i=0;i<q;++i){
		if(res[i])cout << "yes" << "\n";
		else cout << "no" << "\n";
	}
}

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

void bfs(int s);

int n_user,m,*deg;
vector<int> *G; 

int main(void){
	cin>>n_user>>m;
	deg = new int[n_user];
	G = new vector<int>[n_user];
	for(int i=0;i<n_user;i++)
		deg[i] = -1;

	int user1,user2;
	for(int i=0;i<m;i++){
		cin>>user1>>user2;
		G[user1].push_back(user2);
		G[user2].push_back(user1);
	}

	for(int i=0;i<n_user;i++){
		cout<<i<<" : ";
		for(int j=0;j<G[i].size();j++){
			cout<<G[i][j]<<" ";
		}
		cout<<endl;
	}

	for(int i=0;i<n_user;i++) bfs(i);

	int question,s,t;
	cin>>question;
	for(int i=0;i<question;i++){
		cin>>s>>t;
		if(deg[s]!=-1){
			if(deg[s]==deg[t])
				cout<<"yes"<<endl;
			else
				cout<<"no"<<endl;
		}
	}

	delete [] G; delete [] deg;
}

void bfs(int s){
	queue<int> q;
	q.push(s);
	if(deg[s]==-1) deg[s] = s;
	else return ;

	int u;
	while(!q.empty()){
		u = q.front();
		q.pop();
		for(int v=0;v<G[u].size();v++){
			if(deg[G[u][v]]==-1){
				deg[G[u][v]] = deg[u];
				q.push(G[u][v]);
			}
		}
	}
}