Go: package main

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

var scanner = bufio.NewScanner(os.Stdin)

func nextString() string {
	scanner.Scan()
	return scanner.Text()
}

func nextInt() int {
	n, err := strconv.Atoi(nextString())
	if err != nil {
		fmt.Printf("strconv.Atoi failed: %v\n", err)
	}
	return n
}

func newGraph(nNodes int) *Graph {
	g := Graph{}
	g.nodes = make([]*GraphNode, nNodes)
	for iNode := 0; iNode < len(g.nodes); iNode++ {
		g.nodes[iNode] = &GraphNode{}
		g.nodes[iNode].key = GraphNodeKey(iNode)
	}
	return &g
}

var infty = 1000000000

type GraphNodeKey int

// GraphNode is a node for undirected graph
type GraphNode struct {
	key       GraphNodeKey
	adjacents []*GraphNode
	visited   bool
	d, f      int
	parent    *GraphNode
	cc        int
}

func (g *Graph) setAdjacent(l, r GraphNodeKey) {
	g.nodes[l].adjacents = append(g.nodes[l].adjacents, g.nodes[r])
}

// Graph is a undirected graph
type Graph struct {
	nodes []*GraphNode
}

func (g *Graph) initialize() {
	for iNode := 0; iNode < len(g.nodes); iNode++ {
		g.nodes[iNode].visited = false
		g.nodes[iNode].d = infty
		g.nodes[iNode].f = infty
		g.nodes[iNode].parent = nil
		g.nodes[iNode].cc = -1 // 22.3-12
	}
}

type QueueEntry struct {
	node *GraphNode
	next *QueueEntry
}

type Queue struct {
	head, tail *QueueEntry
}

func (q *Queue) enqueue(node *GraphNode) {
	entry := QueueEntry{}
	entry.node = node

	if q.tail == nil {
		q.head = &entry
		q.tail = &entry
		return
	}

	q.tail.next = &entry
	q.tail = q.tail.next
}

func (q *Queue) dequeue() *QueueEntry {
	if q.head == nil {
		return nil
	}
	poped := q.head
	q.head = q.head.next
	if q.head == nil {
		q.tail = nil
	}
	return poped
}

func (q *Queue) isEmpty() bool {
	return q.head == nil
}

var time = 0

func fillDistancesByDepthFirstSearch(g *Graph) {
	g.initialize()
	time = 0
	cc := 0

	for _, u := range g.nodes {
		if !u.visited {
			dfsVisit(g, u, cc)
			cc++
		}
	}
}

func dfsVisit(g *Graph, u *GraphNode, cc int) {
	time++
	u.d = time
	u.cc = cc

	u.visited = true
	for _, v := range u.adjacents {
		if !v.visited {
			v.parent = u
			dfsVisit(g, v, cc)
		}
	}
	time++
	u.f = time
}

func main() {
	scanner.Split(bufio.ScanWords)

	nUsers := nextInt()
	nEdges := nextInt()
	graph := newGraph(nUsers)
	for iEdge := 0; iEdge < nEdges; iEdge++ {
		l := GraphNodeKey(nextInt())
		r := GraphNodeKey(nextInt())
		graph.setAdjacent(l, r)
		graph.setAdjacent(r, l)
	}

	fillDistancesByDepthFirstSearch(graph)

	nQueries := nextInt()
	for iQuery := 0; iQuery < nQueries; iQuery++ {
		l := GraphNodeKey(nextInt())
		r := GraphNodeKey(nextInt())

		// fmt.Println(graph.nodes[l].d, graph.nodes[r].d, graph.nodes[r].f, graph.nodes[l].f)
		if graph.nodes[l].cc == graph.nodes[r].cc {
			fmt.Println("yes")
		} else {
			fmt.Println("no")
		}
	}
}

C++: //
//
//  ad-algoD.cpp - created by us162022
//
#include <iostream>
using namespace std;
int find(int join[],int x){
  if(x==join[x])
    return x;

  return join[x];
}
int main(int argc, char *argv[])
{
  int n,m,join[100000],num[100000],s,t;
  cin >> n >> m;

  for(int i=0; i<n; i++){
    join[i] = i;
    num[i] = 0;
  }

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

    if(s!=t){
      if(num[s] < num[t])
	join[s] = join[t];
      
      else{
	join[t]=join[s];
	if(num[s]==num[t])
	  num[s]++;
      }

    }
  }

  int q;
  cin >> q;

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

    if(find(join,s)==find(join,t))
      cout << "yes\n";

    else
      cout << "no\n";
  }
}

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

#define MAX_N (100005)
int n, m;
vector<int> G[MAX_N];
int Group[MAX_N];

int main()
{
    int id = 0;

    scanf("%d %d", &n, &m);
    
    for ( int i = 0; i < m; i++ ) {
	int s, t;
	scanf("%d %d", &s, &t);
	G[s].push_back(t);
	G[t].push_back(s);
    }

    for ( int i = 0; i < n; i++ ) {
	if ( Group[i] == 0 ) {
	    id++;
	    queue<int> q;
	    q.push(i);
	    Group[i] = id;
	    while ( !q.empty() ) {
		int cur = q.front(); q.pop();
		for ( int j = 0; j < G[cur].size(); j++ ) {
		    int fri = G[cur][j];
		    if ( Group[fri] == 0 ) {
			Group[fri] = id;
			q.push(fri);
		    }
		}
	    }
	}
    }

    int q;
    scanf("%d", &q);

    for ( int i = 0; i < q; i++ ) {
	int s, t;
	scanf("%d %d", &s, &t);
	if ( Group[s] == Group[t] ) {
	    printf("yes\n");
	}
	else {
	    printf("no\n");
	}
    }

    return 0;
}

C++: #include <iostream>
#include <string>
#include <vector>
#include <set>
#include <cstdio>
#include <cstdlib>
#include <algorithm>
#include <functional>
#include <map>
#include <cmath>
#include <queue>
#include <stack>

using namespace std;

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

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


//深さ優先探索を行う
void dfs(int r, int c){

    //スタックを作る
    stack<int> S;

    //スタックに初期頂点を入れる
    S.push(r);

    //color(ID)を初期頂点に代入する
    color[r] = c;


    //スタックが空でない限り行う
    //rと同じIDを入れ続ける
    while(!S.empty()){

        //スタックの一番上を取り出す
        int u = S.top();S.pop();

        //頂点uにつながっている頂点の数だけ繰り返す
        for(int i=0;i<G[u].size();i++){

            //uにつながっている頂点vを取り出す
            int v = G[u][i];

            //もしvがまだIDが決まっていなかったら
            //まだ何も頂点vが触られていなかったら
            //これがないと永遠にループする
            if(color[v] == NIL){

                //color[v]に同じIDを代入する
                color[v] = c;

                //スタックにvを追加する
                S.push(v);
            }
        }
    }
}

// 連結している部分を同じIDにする
void assignColor(){
    int id = 1;

    //どのIDと等しいか初期化する　初期化値はNIL
    for(int i=0;i<n;i++){
        color[i] = NIL;
    }

    //頂点分繰り返す
    for(int u=0;u<n;u++){

        //もし、ある頂点uのIDがまだ決まっていなかったら
        if(color[u] == NIL){

            //深さ優先探索を行う
            dfs(u, id++);
        }
    }
}

int main() {

    int s, t, m, q;

    cin >> n >> m;  //n・・ユーザーの数　　　m・・友達関係の数
    for(int i=0;i<m;i++){
        cin >> s >> t;
        G[s].push_back(t);        //sとtが友人であることを示す
        G[t].push_back(s);
    }

    assignColor();          //連結している部分を同じIDにする

    cin >> q;

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

    return 0;
}

C++: /* 
 * ALDS1_11B.cpp
 */
#include <iostream>
#include <queue>
#include <cstdio>
#include <cstdlib>
#include <cstring>

using namespace std;

using Friends = vector<int>;
using SNS = vector<Friends>;

int n, m, q;
bool visited[100000];
SNS sns;

bool is_friend(int id, int target) {
    if (visited[id])
        return false;

    visited[id] = true;
    for (int i = 0; i < sns[id].size(); ++i) {
        int now = sns[id][i];
        if (is_friend(now, target))
            return true;
    }
    return false;
}

int main(void) {

    scanf("%d %d", &n, &m);
    sns.resize(n);
    for (int i = 0, id, f; i < m; ++i) {
        scanf("%d %d", &id, &f);
        sns[id].push_back(f);
        sns[f].push_back(id);
    }

    scanf("%d", &q);
    for (int i = 0, id, f; i < q; ++i) {
        scanf("%d %d", &id, &f);
        memset(visited, 0, sizeof(bool) * n);
        if (is_friend(id, f))
            printf("yes\n");
        else
            printf("no\n");
    }

    return 0;
}