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 <vector>
#include <algorithm>
#include <deque>
using namespace std;
 
struct status{
    int id;
    vector<int> children{};
    int visited = 0;
    
    void resetVisited(){
        this->visited = 0;
    }
};
 
deque<int> schedule; 
vector<status> graph;

bool bfs(int target_v){
    if (schedule.empty()){
        return false;
    }
    
    int now_v = schedule.front();
    schedule.pop_front();
    
    for(auto& next_v:graph[now_v].children){
        if(graph[next_v].visited) continue;

        if(next_v==target_v) return true;
        graph[next_v].visited = 1;
        schedule.push_back(next_v);
    }
    return bfs(target_v);
}
 

int main(void){cin.tie(0);ios::sync_with_stdio(false);
 
    //setup
    int nb_v; cin >> nb_v;
    int nb_e; cin >> nb_e;
    graph.resize(nb_v);
    for(int i=0; i<nb_e; ++i){
        int v1, v2; cin >> v1 >> v2;
        graph[v1].children.push_back(v2);
        graph[v2].children.push_back(v1);
    }
    for(auto& a:graph){
        sort(a.children.begin(),a.children.end());
    }
    
    //bfs
    int times; cin>>times;
    for(int i=0; i<times; ++i){
        int from_v; cin>>from_v;
        int to_v; cin>>to_v;
        for(auto& a:graph) a.resetVisited();
        schedule = {};
        schedule.push_back(from_v);
        cout<<(bfs(to_v)? "yes\n":"no\n");
    }
}

C++: #include <bits/stdc++.h>

#define REP(i, n) for(int i=0;i<(int)(n);i++)
#define ALL(x) (x).begin(),(x).end()
const int INF = 1e9;

using namespace std;
int n, m;
struct node {
    int parent = -1;
};

int main() {
    cin >> n >> m;
    node person[n];
    REP(i, m){
        int s, t;
        cin >> s >> t;
        if(person[s].parent == -1){
            person[s].parent = s;
            person[t].parent = s;
        }
        else
            person[t].parent = person[s].parent;
    }
    int q;
    cin >> q;
    REP(i, q){
        int s, t;
        cin >> s >> t;
        if(person[s].parent != -1 && person[t].parent != -1  && person[s].parent == person[t].parent)
            cout << "yes" << endl;
        else
            cout << "no" << endl;
    }
    return 0;
}

C++: #include<bits/stdc++.h>
#define ll long long
#define rep(i, n) for(int i=0; i<n; i++)
#define REP(i, n) for(int i=0; i<=n; i++)
#define repr(i, n) for(int i=n; i>0; i--)
#define REPR(i, n) for(int i=n; i>=0; i--)
const ll INF = 1LL<<60;
const int INT_INF = 1e9;
const ll MOD = 1e9+7;
template<class T> void puts(T x) { std::cout << x << std::endl; }
template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }
using namespace std;

struct UnionFind {
    vector<int> parent;

    // Constructor
    UnionFind(int n) {
        parent.resize(n);
        rep(i, n) parent[i] = i;
    }

    // 親を取得
    int root(int x) {
        if (parent[x] == x) {
            return x;
        } else {
            parent[x] = root(parent[x]);
            return parent[x];
        }
    }

    // 二つのデータを同じ木にくっつける
    void connect(int x, int y) {
        int rx = root(x);
        int ry = root(y);

        if (rx == ry) return; // 元から同じ木に属する場合は何もしない
        parent[rx] = ry;
    }

    // 二つのデータが同じ木に属するか判定
    bool check(int x, int y) {
        int rx = root(x);
        int ry = root(y);
        return rx == ry;
    }
};

int main() {
    int n, m; cin >> n >> m;
    UnionFind tree(n);
    rep(i, m) {
        int s, t; cin >> s >> t;
        tree.connect(s, t);
    }
    int q; cin >> q;
    rep(i, q) {
        int s, t; cin >> s >> t;
        cout << (tree.check(s, t) ? "yes" : "no") << endl;
    }
}

C++: #include <cstdio>
#include <vector>
#include <cstring>

bool isFriend(std::vector<int> *a, int s, int t, int *checked, int c){
    int tmp;
    checked[s] = c;
    if(s==t)return true;
    for(int i=0;i<a[s].size();i++){
        tmp = a[s][i];
        if(checked[tmp]>=c)continue;
        if(isFriend(a,tmp,t,checked,c))return true;
    }
    return false;
}

int main(){
    int i, n, m, q, s, t;
    int *checked;
    scanf("%d %d",&n,&m);
    checked = (int *)malloc(sizeof(int)*n);
    memset(checked, 0, sizeof(int)*n);
    std::vector<int> a[n];
    for(i=0;i<m;i++){
        scanf("%d %d",&s,&t);
        a[s].push_back(t);
        a[t].push_back(s);
    }
    scanf("%d",&q);
    for(i=0;i<q;i++){
        scanf("%d %d",&s,&t);
        if(isFriend(a,s,t,checked,i+1))printf("yes\n");
        else printf("no\n");
    }
    return 0;
}