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++: //
//  main.cpp
//  connected_components
//
//  Created by ???????????? on 2017/10/22.
//  Copyright ?? 2017 ????????????. All rights reserved.
//

#include <iostream>
#include <stdio.h>
#include <vector>

using namespace std;

#define n_max 100000
//#define vector_max 100000
#define WHITE 0
#define GRAY 1
#define BLACK 2

bool found;
int n, target, t = 0;
int color[n_max], d[n_max], f[n_max];
vector<int> G[n_max];

void encode_graph_list(){
    int i, id, node, f;
    
    scanf("%d", &n);
    scanf("%d", &node);
    
    for (i = 0; i < node; i++){
        scanf("%d", &id);
        scanf("%d", &f);
        G[id].push_back(f);
    }
    
}

void dfs(int u){
//    found = false;
//    if(target == u) {
//        found = true;
//        return;
//    }
    color[u] = GRAY;
    d[u] = ++t;
    for (int v = 0; v < G[u].size(); v++){
        int t = G[u][v];
        if(color[t] == WHITE){
            dfs(t);
        }
    }
    color[u] = BLACK;
    f[u] = ++t;
}

void dfs_init(int start){
    for(int i = 0; i < n ; i++){
        color[i] = WHITE;
    }
    dfs(start);
}



int main(int argc, const char * argv[]) {
    int qs, i, id;
    encode_graph_list();
    scanf("%d", &qs);
    for (i = 0; i < qs; i++){
        scanf("%d %d", &id, &target);
        dfs_init(id);
        if(color[target] == BLACK) cout << "yes" <<endl;
        else cout << "no"<<endl;
    }
//    std::cout << "Hello, World!\n";
    return 0;
}

C++: #include <stdio.h>
#include <stdint.h>
#include <iostream>
#include <fstream>
#include <cassert>
#include <climits>

#include <queue>
#include <map>
#include <utility>
using namespace std;

#define DEBUG (0)

#if DEBUG
#define DPRINTF(...) do { \
    printf("D(L%d) %s: ", __LINE__, __func__); \
    printf(__VA_ARGS__); \
  } while (false)
#else
#define DPRINTF(...)
#endif // DEBUG

/*
   Memory limitaion; 64 MiB
   # of users: 2 <= n <= 100 K
   # of relations: 0 <= m <= 100 K

   Data size:
    1) for relations (maybe hash table):
      100K * 4Byte * 2 (bi-direction) = 800K

    2) for visited flag:
      100K * 1Byte = 100K
*/

multimap<int, int> g_relations;

void setRelations(int id0, int id1)
{
  g_relations.insert(make_pair(id0, id1));
  g_relations.insert(make_pair(id1, id0));
}


string isReachable(int num, int start, int end)
{

  bool isVisited[num];

  for (int i = 0; i < num; i++)
    isVisited[i] = false;

  queue<int> que;
  que.push(start);

  while (!que.empty())
  {
    int id = que.front();
    que.pop();
    isVisited[id] = true;

    // if no path from "id", try next
    if (g_relations.count(id) == 0)
      continue;

    // check all pathes connected from "id"
    std::multimap<int, int>::iterator it;
    for (it = g_relations.equal_range(id).first; it != g_relations.equal_range(id).second; it++)
    {
      int &dst = it->second;

      if (dst == end)
        return string("yes");

      if (!isVisited[dst])
        que.push(dst);

    }
  }

  return string("no");
}


void printAllRelations()
{
#if DEBUG
  for (auto& m: g_relations)
    DPRINTF("%d %d\n", m.first, m.second);
#endif // DEBUG
}

int main()
{
  int n, m;
  cin >> n >> m; cin.ignore();

  DPRINTF("number of users %d\n", n);
  DPRINTF("number of relations %d\n", m);

  for (int i = 0; i < m; i++)
  {
    int s, t;
    cin >> s >> t; cin.ignore();

    setRelations(s, t);
  }

  printAllRelations();

  int q;
  cin >> q; cin.ignore();

  for (int i = 0; i < q; i++)
  {
    int s, t;
    cin >> s >> t; cin.ignore();

    DPRINTF("src %d  dst %d\n", s, t);

    cout << isReachable(n, s, t) << endl;
  }

  return 0;

}

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

static const int MAX =100000;
static const int NIL=-1;

int n;
vector<int> G[MAX];
int color[MAX];

void dsf(int r,int c){
  stack<int> S;
  S.push(r);
  color[r]=c;
  while(!S.empty()){
    int u=S.top();S.pop();
    for(int i=0;i<G[u].size();i++){
      int v=G[u][i];
      if(color[v]==NIL){
        color[v]=c;
        S.push(v);
      }
  }
}

void assignColor(){
  int id=1;
  for(int i=0;i<n;i++) color[i]=NIL;
  for(int u=0;u<n;u++){
    if(color[u]==NIL)dfs(u,id++);
  }
}

int main (){
  int s,t,m,q;
  cin >> n >>m;
  for(int i=0;i<m;i++){
    cin >> s >> t;
    G[s].push_back(t);
    G[t].push_back(s);
  }

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

  return 0;
}

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

static const int MAX = 100000;
static const int NIL = -1;


int n;
vector<int> G[MAX];
int color[MAX];

void dfs(int r, int c){
    static<int> S;
    S.push(r);
    color[r] = c;
    while (!S.empty()){
        int u = S.top(); S.pop();
        for(int i = 0; i < G[u].size(); i++){
            int v = G[u][i];
            if (color[v] == NIL){
                color[v] = c;
                S.push(v);
            }
        }
    }
}

void assignColor(){
    int id = 1;
    for(int i = 0; i < n; i++) color[i] = NIL;
    for(int u = 0; u < n; u++){
        if(color[u] == NIL) dfs(u, id++);
    }
}

int main(){
    int s, t, m, q;

    cin >> n >> m;

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

    assignColor();

    cin >> q;

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

    return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;
int f[100001],n,m,q;
int getf(int u){
	if(u==f[u])return u;
	else return f[u]=getf(f[u]);
	}
int merge(int u,int v){
	int t1=getf(u);
	int t2=getf(v);
	if(t1!=t2){
		f[t2]=t1;
			}
}	
int main(){
	int v1,v2;
	scanf("%d%d",&n,&m);
	for(int i=0;i<n;i++)f[i]=i;
	for(int i=0;i<m;i++){
		scanf("%d%d",&v1,&v2);
		merge(v1,v2); 
	}
	scanf("%d",&q);
	for(int i=0;i<q;i++){
		scanf("%d%d",&v1,&v2);
		if(getf(v1)==getf(v2))printf("yes\n");
		else printf("no\n");
	}
}