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 <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <math.h>

//============================================================================
#define pred(x) ((x)-1)
#define succ(x) ((x)+1)
#define True    1
#define False   0
//#define max(x,y) (((x)<(y))?y:x)
//============================================================================
#define MAX_BUFFER  102
struct graph{
    long id;
    unsigned char hands;
    unsigned char d;
    struct graph *gary[20];
};
typedef struct graph TGraph;
//============================================================================
unsigned char usedflags[100000];
short Stamp;
//============================================================================
void prnnode(TGraph *gptr)
{
    short j;

    printf("node=%d, hands=%d, ", gptr->id, gptr->hands);
    for (j=0; j<gptr->hands; j++)
        printf("%d ", gptr->gary[j]->id);
    printf("\n");
}
//----------------------------------------------------------------------------
void setflag(long n)
{
    usedflags[n>>3] |= 1<<(n & 0x7);
}
//----------------------------------------------------------------------------
int getflag(long n)
{
    return (usedflags[n>>3] & 1<<(n & 0x7));
}
//----------------------------------------------------------------------------
short search(TGraph *gptr, TGraph *fptr)
{
    short i;

    if (gptr==fptr)
        return True;
    //setflag(gptr->id);
    usedflags[gptr->id]=True;
#ifdef DESKTOP
   //prnnode(gptr);
#endif
    for (i=0; i<gptr->hands; i++){
        //if (!getflag(gptr->gary[i]->id)){
        if (!usedflags[gptr->gary[i]->id]){
            if (search(gptr->gary[i], fptr))
                return True;
        }
    }
    /*for (i=0; i<fptr->hands; i++){
        if (search(fptr->gary[i], gptr))
            return True;
    }*/
    return False;
}
//----------------------------------------------------------------------------
void addfrend(TGraph *gptr, TGraph *fptr)
{
    //TGraph **ptr; short i;

    //ptr=malloc(succ(gptr->hands)*sizeof(TGraph**));
    //for (i=0; i<gptr->hands; i++)
    //    ptr[i]=gptr->gary[i];

    //if (gptr->gary)
    //    free(gptr->gary);
    //gptr->gary=ptr;
    gptr->gary[gptr->hands++]=fptr;
}
//----------------------------------------------------------------------------
void filld(TGraph *gptr, long n)
{
    long i;

    for (i=0; i<n; i++)
        gptr[i].d=0;
}
//----------------------------------------------------------------------------
void input(FILE *file)
{
    long i,j,n,m,q,s,t; char buffer[20]; TGraph *graphary,*gptr,*fptr;

    fgets(buffer, sizeof(buffer)-1, file);
    sscanf(buffer, "%ld %ld", &n, &m);
    graphary=malloc(n*sizeof(TGraph));
    memset(graphary, 0, sizeof(TGraph));
    for (i=Stamp=0; i<m; i++){
        fscanf(file, "%ld", &s);
        fscanf(file, "%ld", &t);
        gptr=&graphary[s];
        fptr=&graphary[t];
        gptr->id=s;
        fptr->id=t;
        addfrend(gptr, fptr);
        addfrend(fptr, gptr);
    }
#ifdef DESKTOP
    for (i=m=0; i<n; i++){
        //prnnode(&graphary[i]);
        if (m<graphary[i].hands)
            m=graphary[i].hands;
    }
    setflag(0);
    printf("max hands = %d, %d\n", m, getflag(0));
#endif
    fscanf(file, "%ld", &q);
    for (i=0; i<q; i++){
        fscanf(file, "%ld", &s);
        fscanf(file, "%ld", &t);
        //filld(graphary, n);
        //memset(usedflags, 0, succ(n>>3));
        memset(usedflags, 0, n);
        printf("%s\n", (search(&graphary[s], &graphary[t]))?"yes":"no");
    }
}
//============================================================================
#ifndef DESKTOP
int main()
{
    input(stdin);
    return 0;
}
#endif

C++: #include <iostream>
#include <vector>
#include <unordered_set>
#include <algorithm>
//#include "../cxx-prettyprint/prettyprint.hpp"

std::vector<std::vector<int>> Adj;
std::unordered_set<int> nodes_visited;

bool is_reachable(int s, int t) {
    if (std::find(Adj[s].begin(), Adj[s].end(), t) != Adj[s].end()) {
        return true;
    } else if (nodes_visited.find(s) != nodes_visited.end()) {
        return false;
    } else {
        nodes_visited.insert(s);
        bool value = std::any_of(Adj[s].begin(), Adj[s].end(),
                                 [&](int next){return is_reachable(next, t);});
        return value;
    }
}

int main() {
    int n, m;
    std::cin >> n >> m;

    Adj.assign(n, std::vector<int>());
    for (int i = 0; i < m; i++) {
        int s, t;
        std::cin >> s >> t;
        Adj[s].push_back(t);
        Adj[t].push_back(s);
    }

    int q;
    std::cin >> q;
    for (int i = 0; i < q; i ++) {
        int s, t;
        std::cin >> s >> t;
        std::cout << (is_reachable(s, t) ? "yes" : "no") << std::endl;
    }
}