Go: package main

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

// I/O
type Scanner struct {
	sc *bufio.Scanner
}

func NewScanner() *Scanner {
	sc := bufio.NewScanner(os.Stdin)
	sc.Split(bufio.ScanWords)
	sc.Buffer(make([]byte, 1024), int(1e+9))
	return &Scanner{sc}
}

func (s *Scanner) nextStr() string {
	s.sc.Scan()
	return s.sc.Text()
}

func (s *Scanner) nextInt() int {
	i, e := strconv.Atoi(s.nextStr())
	if e != nil {
		panic(e)
	}
	return i
}

func (s *Scanner) nextFloat() float64 {
	f, e := strconv.ParseFloat(s.nextStr(), 64)
	if e != nil {
		panic(e)
	}
	return f
}

func (s *Scanner) nextRuneSlice() []rune {
	return []rune(s.nextStr())
}

func (s *Scanner) nextIntSlice(n int) []int {
	res := make([]int, n)
	for i := 0; i < n; i++ {
		res[i] = s.nextInt()
	}
	return res
}

func (s *Scanner) nextFloatSlice(n int) []float64 {
	res := make([]float64, n)
	for i := 0; i < n; i++ {
		res[i] = s.nextFloat()
	}
	return res
}

// Arithmetic
func max(nums ...int) int {
	m := nums[0]
	for _, i := range nums {
		if m < i {
			m = i
		}
	}
	return m
}

func min(nums ...int) int {
	m := nums[0]
	for _, i := range nums {
		if m > i {
			m = i
		}
	}
	return m
}

func abs(x int) int {
	if x > 0 {
		return x
	}
	return -x
}

func pow(x, y int) int {
	res := 1
	for i := 0; i < y; i++ {
		res *= x
	}
	return res
}

func ceil(a, b int) int {
	if a%b == 0 {
		return a / b
	} else {
		return a/b + 1
	}
}

// Sort
type RuneSlice []rune

func (a RuneSlice) Len() int           { return len(a) }
func (a RuneSlice) Less(i, j int) bool { return a[i] < a[j] }
func (a RuneSlice) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }

// Main
func main() {
	sc := NewScanner()
	wtr := bufio.NewWriter(os.Stdout)
	N, M := sc.nextInt(), sc.nextInt()
	G := make([][]int, N)
	for i := 0; i < M; i++ {
		s, t := sc.nextInt(), sc.nextInt()
		G[s] = append(G[s], t)
		G[t] = append(G[t], s)
	}
	group := make([]int, N)
	for i := 0; i < N; i++ {
		group[i] = -1
	}
	var dfs func(cur int)
	dfs = func(cur int) {
		for i := 0; i < len(G[cur]); i++ {
			next := G[cur][i]
			if group[next] == -1 {
				group[next] = group[cur]
				dfs(next)
			}
		}
	}
	num := 0
	for i := 0; i < N; i++ {
		if group[i] == -1 {
			group[i] = num
			dfs(i)
			num++
		}
	}

	Q := sc.nextInt()
	for i := 0; i < Q; i++ {
		s, t := sc.nextInt(), sc.nextInt()
		if group[s] == group[t] {
			fmt.Fprintln(wtr, "yes")
		} else {
			fmt.Fprintln(wtr, "no")
		}
	}
	wtr.Flush()
}

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){
    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++); //それぞれの連結な部分グラフに番号（色）を与える
    }
}

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 <cstdio>
#include <iostream>
#include <vector>
#include <stack>
using namespace std;

static const int MAX_NUM = 100000;

void DFS(int group[], vector<int> vec[], int gnum, int start)
{
    stack<int> S;
    int target;
    int i;
    int tmp;

    S.push(start);
    group[start] = gnum;

    while (S.empty() == false) {
        target = S.top();
        for (i = 0; i < vec[target].size(); i++) {
            if (group[vec[target][i]] == 0) {
                group[vec[target][i]] = gnum;
                S.push(vec[target][i]);
                break;
            }
        }
        if (i == vec[target].size()) {
            S.pop();
        }
    }

    return;
}

void SetupGroup(int group[], vector<int> vec[], int elemnum)
{
    int i;
    int gnum = 0;

    for (i = 0; i < elemnum; i++) {
        if (group[i] == 0) {
            ++gnum;
            DFS(group, vec, gnum, i);
        }
    }
}

int main(void)
{
    int i;
    int num;
    int elemnum;
    int qnum;
    int ele1, ele2;
    vector<int> vec[MAX_NUM];
    int group[MAX_NUM] = {0};

    scanf("%d %d", &elemnum, &num);
    for (i = 0; i < num; i++) {
        scanf("%d %d", &ele1, &ele2);
        vec[ele1].push_back(ele2);
        vec[ele2].push_back(ele1);
    }

    SetupGroup(group, vec, elemnum);

    scanf("%d", &qnum);
    for (i = 0; i < qnum; i++) {
        scanf("%d %d", &ele1, &ele2);
        if (group[ele1] == group[ele2]) {
            printf("yes\n");
        }
        else {
            printf("no\n");
        }
    }
}

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

bool x[10000][10000];
bool y[10000];

int main(){
	int n,m,a,b,q;
	cin>>n>>m;
	for(int i=0;i<m;i++){
		cin>>a>>b;
		x[a][b]=true;
		x[b][a]=true;
	}
	cin>>q;
	for(int h=0;h<q;h++){
		cin>>a>>b;
		memset(y,0,sizeof(y));
		y[a]=true;
		for(int i=0;i<100;i++){
			for(int j=0;j<n;j++){
				if(y[j]==true){
					for(int k=0;k<n;k++){
						if(x[j][k]==true){
							y[k]=true;
						}
					}
				}
				if(y[b]==true){
					cout<<"yes"<<endl;
					goto Exit;
				}
			}
		}
		cout<<"no"<<endl;
		Exit:;
	}
	return 0;
}

C++: #include <cstdio>
#include <iostream>
#include <algorithm>
#include <vector>
using namespace std;
const int maxx=100010;

int pre[maxx];

int find(int u){
	while(pre[u]!=u) u=find(pre[u]);
	return u;
}

int main(){
	int n,m,x,y;
	cin>>n>>m;
	for(int i=0;i<n;i++) pre[i]=i;
//	for(int i=0;i<n;i++) cout<<i<<" ";cout<<endl;
//	for(int i=0;i<n;i++) cout<<pre[i]<<" ";cout<<endl;
	while(m--){
		cin>>x>>y;
		pre[find(x)]=find(y);
//		for(int i=0;i<n;i++) cout<<i<<" ";cout<<endl;
//		for(int i=0;i<n;i++) cout<<pre[i]<<" ";cout<<endl;
	}
	cin>>m;
	while(m--){
		cin>>x>>y;
		if(find(x)==find(y)) cout<<"yes"<<endl;
		else cout<<"no"<<endl;
	}
	return 0;
}

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

using namespace std;

const int N = 100000;
int n;
int color[N];
vector<int> v[N];

bool isFriend(int s, int t){
  for(int i=0;i<n;i++) color[i] = 0;
  queue<int> q;
  int x;
  q.push(s);
  while(!q.empty()){
    x = q.front(); q.pop();
    for(int i=0;i<v[x].size();i++){
      int y = v[x][i];
      if(y == t) return true;
      if(color[y] == 0){
        q.push(y);
        color[y] = 1;
      }
    }
  }
  return false;
}

int main(){
  int m;
  cin >> n >> m;
  int s, t;
  for(int i=0;i<m;i++){
    cin >> s >> t;
      v[s].push_back(t);
      v[t].push_back(s);
  }
  int qn; cin >> qn;
  for(int i=0;i<qn;i++){
    cin >> s >> t;
    if(isFriend(s, t)) cout << "yes" << endl;
    else cout << "no" << endl;
  }
  return 0;
}

C++: #include<cstdio>
#include<cstdlib>
#include<cstring>
#include<vector>
using namespace std;
vector<int> neighbors[100005];
//why choose vector, not matrix
int root[100005];

void dfs(int i, int start){
	root[i] = start;
	//visut all i's neighbors
	//c++ 11
	for(int j : neighbors[i]){
		if(root[j]==-1){
			dfs(j,start);
		}
	}
}

int main(){
	int n,m,x,y;
	memset(root, -1, sizeof(root));
	scanf("%d %d",&n,&m);
	for(int i = 0; i<m; i++){
		scanf("%d %d",&x,&y)
		neighbors[x].push_back(y);
		neighbors[y].push_back(x);
	}
	for(int i = 0; i<n; i++){
		if(root[x]= -1){
			dfs(i,i);
		}
	}
	scanf("%d", &q);
	for(int i = 0; i<q; i++){
		scanf("%d %d",&x, &y);
		if(root[x] = root[y]){
			printf("Yes\n");
		}else printf("no\n");
	}
	return 0;
}