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++: #include <iostream>
#include <string>
#include <vector>
#include <queue>
#include <unordered_set>

using namespace std;

template<typename ISTREAM>
void readGraphListFromFriendInfo(ISTREAM& cin, vector< unordered_set<int> >& friendGraphList) {
	size_t n;
	cin >> n;
	friendGraphList = vector< unordered_set<int> >(n);
	size_t m;
	cin >> m;
	for (size_t i = 0; i != m; ++i) {
		size_t s,t;
		cin >> s >> t;
		friendGraphList.at(s).insert(t);
		friendGraphList.at(t).insert(s);
	}
}

bool checkFriendAccesibility(const vector<unordered_set<int>>& graphList, const int s, const int t) {
	const size_t n = graphList.size();
	queue<int> nodeQueue;
	//vector<int> distanceList(n, -1);//
	vector<bool> isVisitList(n, false);//距離に興味はないのでbool
	nodeQueue.push(s);
	while (!nodeQueue.empty()) {
		const int id = nodeQueue.front();
		isVisitList.at(id).flip();//trueにする
		const auto& adjList = graphList.at(id);
		for (const auto& adjID : adjList) {
			if (isVisitList.at(adjID))continue;
			if (graphList.at(adjID).find(id) == graphList.at(adjID).end())continue;
			if (adjID == t)return true;
			nodeQueue.push(adjID);
		}
		nodeQueue.pop();
	}
	return false;
}

template<typename T>
void execute(T& cin) {
	vector< unordered_set<int> > graphList;
	readGraphListFromFriendInfo(cin, graphList);
	int q, s, t;
	cin >> q;
	for (int i = 0; i != q; ++i) {
		cin >> s >> t;
		cout << (checkFriendAccesibility(graphList, s, t) ? "yes" : "no") << endl;
	}
}

int main()
{
	execute(cin);
	return 0;
}

C++: #include <cstdio>
#include <cstdlib>

typedef struct{
    int height;
    int parent;
}node;

node* nd;

int top(int key){
    if(nd[key].parent == -1)return key;
    else return top(nd[key].parent);
}

void join(int s, int t){
    int a = top(s);
    int b = top(t);
    if(a==b)return;
    if(nd[a].height<nd[b].height){
        nd[a].parent = b;
    }else{
        nd[b].parent = a;
        if(nd[a].height==nd[b].height)nd[a].height++;
    }
    return;
}

bool isFriend(int s, int t){
    if(top(s)==top(t))return true;
    else return false;
}

int main(){
    int i, n, m, s, t, q;
    scanf("%d %d",&n,&m);

    nd = (node *)malloc(sizeof(node)*n);

    for(i=0;i<n;i++){
        nd[i].height = 1;
        nd[i].parent = -1;
    }

    for(i=0;i<m;i++){
        scanf("%d %d",&s,&t);
        join(s,t);
    }

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

C++: #include <iostream>
#include <cstdio>
#include <vector>
#define MAX_NODE 100000
#define MAX_EDGE 100000

using namespace std;

int n;//num of nodes
int e;//num of edges
int group[MAX_NODE+1];//0オリジン
int q;

bool neighbor_a[MAX_NODE+1];
vector<int> neighbor_v[MAX_NODE+1];//連接リスト

void addNeighbor(int num,int ori){//oriを含む連結グラフをつくる
	int ngb;
	if(neighbor_v[num].size() == 0){
		neighbor_a[neighbor_v[num][num]] = true;
		neighbor_v[ori].push_back(neighbor_v[num][num]);
		return;
	}
	for(int i = 0;i < neighbor_v[num].size();i++){//深さ優先探索
		if(neighbor_a[neighbor_v[num][i]] == false){
			neighbor_a[neighbor_v[num][i]] = true;
			neighbor_v[ori].push_back(neighbor_v[num][i]);
			ngb = neighbor_v[num][i];
			addNeighbor(ngb,ori);
		}
	}
}


int main(){
	int v_1,v_2;
	scanf("%d %d",&n,&e);

	for(int i = 0;i < e;i++){
		scanf("%d %d",&v_1,&v_2);
		neighbor_v[v_1].push_back(v_2);
		neighbor_v[v_2].push_back(v_1);
	}

	for(int i  = 0;i < n;i++) group[i] = -1;

	for(int i = 0;i < n;i++){
		if(group[i] != -1) continue;

		for(int j = 0;j < n;j++) neighbor_a[j] = false;//iの連接配列

		addNeighbor(i,i);

		for(int j = 0;j < neighbor_v[i].size();j++) group[neighbor_v[i][j]] = i;

	}
	//for(int i = 0;i < n;i++) cout <<group[i] <<" ";
	//for(int i = 0;i < neighbor_v[5].size();i++) cout <<neighbor_v[5][i] <<" ";

	scanf("%d",&q);

	for(int i = 0;i < q;i++){
		scanf("%d %d",&v_1,&v_2);
		if(group[v_1] == group[v_2]) cout <<"yes" <<endl;
		else cout <<"no" <<endl;
	}
	return 0;
}