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 <algorithm>
#include <unordered_set>

using namespace std;

template<typename ISTREAM>
void readGraphListFromFriendInfo(ISTREAM& cin, vector< vector<int> >& friendGraphList) {
	size_t n;
	cin >> n;
	friendGraphList = vector< vector<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).push_back(t);
		friendGraphList.at(t).push_back(s);
	}
	for (auto& v : friendGraphList)v.shrink_to_fit();
}

bool checkFriendAccesibility(const vector<vector<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;
			const vector<int>& idList = graphList.at(adjID);
			if ( find(idList.begin(), idList.end(), id ) == idList.end())continue;
			if (adjID == t)return true;
			nodeQueue.push(adjID);
		}
		nodeQueue.pop();
	}
	return false;
}

template<typename T>
void execute(T& cin) {
	vector< vector<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 <algorithm>
#include <iostream>
#include <set>
#include <string>
#include <vector>

class TUser
{
	std::set<TUser*> FAccessible;
public:
	void AddFriend(TUser* Friend);
	bool IsAccesible(TUser* Target)const;
};

void TUser::AddFriend(TUser* Friend)
{
	if(IsAccesible(Friend)) return;
	FAccessible.insert(Friend);
	for(std::set<TUser*>::iterator It = FAccessible.begin(); It != FAccessible.end(); ++It){
		(*It)->AddFriend(Friend);
		Friend->AddFriend(*It);
	}
}

bool TUser::IsAccesible(TUser* Target)const
{
	return (FAccessible.find(Target) != FAccessible.end());
}

class TSns
{
	std::vector<TUser> FUsers;
public:
	TSns(int UserCount);
	
	void AddLink(int UserA, int UserB);
	std::string IsAccesible(int UserA, int UserB);
};

TSns::TSns(int UserCount)
: FUsers(UserCount)
{
}

void TSns::AddLink(int UserA, int UserB)
{
	FUsers[UserA].AddFriend(&FUsers[UserB]);
}

std::string TSns::IsAccesible(int UserA, int UserB)
{
	if(FUsers[UserA].IsAccesible(&FUsers[UserB])) return "yes";
	return "no";
}

int main()
{
	int UserCount;
	std::cin >> UserCount;
	TSns Sns(UserCount);
	
	int LinkCount;
	std::cin >> LinkCount;
	for(int i = 0; i < LinkCount; ++i){
		int UserA, UserB;
		std::cin >> UserA >> UserB;
		Sns.AddLink(UserA, UserB);
	}
	
	int QuestionCount;
	std::cin >> QuestionCount;
	for(int i = 0; i < QuestionCount; ++i){
		int UserA, UserB;
		std::cin >> UserA >> UserB;
		std::cout << Sns.IsAccesible(UserA, UserB) << std::endl;
	}
	return 0;
}

C++: #include <iostream>
#include <vector>
#include <stack>
#include <string>
#include <string.h>
using namespace std;
vector< vector<int> > v(10001);
//stack<int> st;
int n, m;
bool f[10001] = { false };

int main() {
	cin >> n >> m;
	for (int i = 0;i < m; i++) {
		int a, b; cin >> a >> b;
		v[a].push_back(b);
		v[b].push_back(a);
	}
	int q; cin >> q;
	vector<string> Ans(q);
	for (int i = 0; i < q; i++) {
		int s, t; cin >> s >> t;

		stack<int> st;
		st.push(s);
		while (!st.empty()) {
			int a = st.top(); st.pop();
			f[a] = true;
			if (a == t) { break; }
			for (int i = 0; i < v[a].size();i++) {
				if (f[v[a][i]] == false) {
					st.push(v[a][i]);
				}
			}
		}
		if (f[t] == true) { Ans[i] = "yes"; memset(f, false, n); }
		else { Ans[i] = "no"; }
	
	}
	for (int i = 0; i < q;i++) {
		cout << Ans[i] << endl;
	}
}