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<queue>
#include<vector>
using namespace std;
#define MAX_N 500000
vector<int>X[MAX_N]; queue<int>Q;
int COLOR[MAX_N], N, M, Q1, A, B;
int main() {
    cin >> N >> M;
    for (int i = 0; i < M; i++) { cin >> A >> B; X[A].push_back(B); X[B].push_back(A); }
    int cnt = 0;
    for (int i = 0; i < N; i++) {
        if (COLOR[i] == 0) {
            Q.push(i); cnt++; COLOR[i] = cnt;
            while (!Q.empty()) {
                int a = Q.front(); Q.pop();
                for (int j = 0; j < X[a].size(); j++) {
                    int to = X[a][j];
                    if (COLOR[to] == 0) {
                        COLOR[to] = cnt;
                        Q.push(to);
                    }
                }
            }
        }
    }
    cin >> Q1;
    for (int i = 0; i < Q1; i++) {
        cin >> A >> B;
        if (COLOR[A] == COLOR[B]) {
            cout << "yes" << endl;
        }
        else {
            cout << "no" << endl;
        }
    }
    return 0;
}

C++: #include<iostream>    // ???????????????????§?????????£????????????
using namespace std;
#include<stdio.h>

typedef struct fox { int val; fox *prev, *next; } fox_t;

fox_t* init()
{
	fox_t* head = new fox_t;
	head->val = -1;
	head->next = NULL;
	head->prev = NULL;
	return head;
}

fox_t* make_tail(fox_t* head)
{
	fox_t* tail = new fox_t;
	tail->val = -1;
	head->next = tail;
	tail->next = NULL;
	tail->prev = head;
	return tail;
}

bool isEmpty(fox_t* head, fox_t* tail)
{
	return (head->next == tail ? true : false);
}

fox_t* b_insert(fox_t* tail, int data)
{
	fox_t* fox = new fox_t;
	fox->val = data;
	fox->prev = tail->prev;
	fox->next = tail;
	tail->prev->next = fox;
	tail->prev = fox;
	return fox;
}

void f_show(fox_t* head, fox_t* tail)
{
	cout << "----??????????????¨???----" << endl;
	fox_t* fox = head->next;

	while(fox != tail){
		cout << fox->val;
		fox = fox->next;
		if(fox != tail) cout << " ";
	};
	cout << endl;
	return;
}
// docking.
void unionfox(fox_t* head1, fox_t* tail1, fox_t* head2, fox_t* tail2)
{
	if(isEmpty(head1, tail1) || isEmpty(head2, tail2)) return;
	fox_t* fox = tail1->prev;
	cout << tail1->prev << endl;
	cout << head2->next << endl;
	fox->next = head2->next;
	head2->next->prev = fox;
	tail2->prev->next = tail1;
	tail1->prev = tail2->prev;
	delete tail2;
	delete head2;
}

int main()
{
	int color[1001];
	int c_num = 0;
	fox_t* foxhead[1001];
	fox_t* foxtail[1001];
	int i, n, m, q;
	int a, b;
	cin >> n >> m;
	for(i = 0; i < n; i++){
		color[i] = -1;
		foxhead[i] = init();
		foxtail[i] = make_tail(foxhead[i]);
	}
	int c_a, c_b, val;

	for(i = 0; i < m; i++){
		cin >> a >> b;
		c_a = color[a];
		c_b = color[b];
		if(c_a < c_b) swap(a, b);  // color[a] >= color[b].
		if(c_a < 0 && c_b < 0){
			color[a] = c_num; color[b] = c_num;
			b_insert(foxtail[c_num], a);
			b_insert(foxtail[c_num], b);
			c_num++;
		}else if(c_b < 0){
			color[b] = c_a;
			b_insert(foxtail[c_a], b);
		}else if(c_a == c_b){
			continue;
		}else{ // color[a] != color[b].
			// b????????¬?????????color???a???color??????????????§???union?????????
			fox_t* fox = foxhead[c_b]->next;
			while(fox != foxtail[c_b]){
				val = fox->val;
				color[val] = c_a;
				fox = fox->next;
			};
			unionfox(foxhead[c_a], foxtail[c_a], foxhead[c_b], foxtail[c_b]);
		}
	}
	cin >> q;
	for(i = 0; i < q; i++){
		cin >> a >> b;
		if(color[a] >= 0 && (color[a] == color[b])){
			printf("yes\n"); }else{ printf("no\n"); 
			}
	}
	return 0;
}

C++: #include<iostream>
#include<queue>
#include<vector>
#include<cstring>
using namespace std;
queue<int>Q; int x[200000], n, m, q, s, t; vector<int>a[100000];

bool bfs(int A,int B) {
	memset(x, 0, sizeof(x));
	x[A] = 1;
	Q.push(A);
	while (!Q.empty()) {
		for (int k = 0; k < a[Q.front()].size(); k++) {
			if (x[a[Q.front()][k]] == 0) { 
				Q.push(a[Q.front()][k]); x[a[Q.front()][k]] = 1; 
			}
		}
		Q.pop();
	}
	if (x[B] == 1) {
		return true;
	}
	else {
		return false;
	}
}

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

C++: #include <stdio.h>
#include <iostream>
#include <utility>
#include <algorithm>
#include <math.h>
#include <vector>
#include <list>
#define N 100000
using namespace std;
int n;
list<int> adj[N];
bool f[N];

bool PathExists(int u, int v)
{
    f[u] = true;
    if (u == v)
        return true;
    list<int>::iterator itr = adj[u].begin();
    while (itr != adj[u].end())
    {
        if (!f[*itr] && PathExists(*itr, v))
            return true;
        itr++;
    }
    return false;
}

int main()
{
    int m, u, v, q;
    cin >> n >> m;

    for (int i = 0; i < m; i++)
    {
        cin >> u >> v;
        adj[u].push_back(v);
        adj[v].push_back(u);
    }

    cin >> q;
    for (int i = 0; i < q; i++)
    {
        for (int j = 0; j < n; j++)
            f[j] = false;
        cin >> u >> v;
        if (PathExists(u, v))
            printf("yes\n");
        else
            printf("no\n");
    }

    return 0;
}