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 <stdint.h>
#include <iostream>
#include <fstream>
#include <cassert>
#include <climits>

#include <cstring>
#include <queue>
#include <map>
#include <utility>
using namespace std;

#define DEBUG (0)

#if DEBUG
#define DPRINTF(...) do { \
    printf("D(L%d) %s: ", __LINE__, __func__); \
    printf(__VA_ARGS__); \
  } while (false)
#else
#define DPRINTF(...)
#endif // DEBUG

/*
   Memory limitaion; 64 MiB
   # of users: 2 <= n <= 100 K
   # of relations: 0 <= m <= 100 K

   Data size:
    1) for relations (maybe hash table):
      100K * 4Byte * 2 (bi-direction) = 800K

    2) for visited flag:
      100K * 1Byte = 100K
*/

const int kMaxUsers = 100 * 1000;

bool g_isVisited[kMaxUsers]; // 100 KiB
vector<int> g_list[kMaxUsers];


void setRelations(int start, int end)
{
  g_list[start].push_back(end);
  g_list[end].push_back(start);
}


string isReachable(int num, int start, int end)
{

  // init
  memset(g_isVisited, 0, num);

  queue<int> que;
  que.push(start);

  while (!que.empty())
  {
    int id = que.front();

    que.pop();
    g_isVisited[id] = true;

    for (int j = 0; j < g_list[id].size(); j++)
    {
      const int &next = g_list[id][j];

      if (next == end)
        return string("yes");

      if (!g_isVisited[next])
        que.push(next);
    }
  }

  return string("no");
}


void printAllRelations()
{
#if DEBUG
  for (int i = 0; i < kMaxUsers; i++)
  {
    for (int j = 0; j < g_list[i].size(); j++)
    {
      if (j == 0)
        DPRINTF("%d", i);

      printf(" %d", g_list[i][j]);

      if (j == g_list[i].size() - 1)
        cout << endl;
    }
  }
#endif // DEBUG
}


int main()
{
  int n, m;
  cin >> n >> m; cin.ignore();

  DPRINTF("number of users %d\n", n);
  DPRINTF("number of relations %d\n", m);

  for (int i = 0; i < m; i++)
  {
    int s, t;
    cin >> s >> t; cin.ignore();

    setRelations(s, t);
  }

  printAllRelations();

  int q;
  cin >> q; cin.ignore();

  for (int i = 0; i < q; i++)
  {
    int s, t;
    cin >> s >> t; cin.ignore();

    DPRINTF("src %d  dst %d\n", s, t);

    cout << isReachable(n, s, t) << endl;
  }

  return 0;

}

C++: #include <iostream>
#include <algorithm>
#include <cstdio>
#include <vector>
#include <stack>
using namespace std;
#define maxn 10005
#define NTL -1
int color[maxn];
vector<int> g[maxn];
stack<int> s;
int n,m,q;
void tarjan(int r,int c)
{
    s.push(r);
    color[r]=c;
    while(!s.empty())
    {
        int u=s.top();
        s.pop();
        for(int i=0;i<g[u].size();i++)
        {
            int v=g[u][i];
            if(color[v]==NTL)
            {
                color[v]=c;
                s.push(v);
            }
        }
    }
}
void assigncolor()
{
    int id=0;
    for(int i=0;i<n;i++)
        color[i]=NTL;
    for(int u=0;u<n;u++)
    {
        if(color[u]==NTL)
            tarjan(u,id++);
    }
}

int main()
{
    scanf("%d%d",&n,&m);
    for(int i=0;i<m;i++)
    {
        int u,v;
        scanf("%d%d",&u,&v);
        g[u].push_back(v);
        g[v].push_back(u);
    }
    assigncolor();
    scanf("%d",&q);
    while(q>0)
    {
        q--;
        int u,v;
        scanf("%d%d",&u,&v);
        if(color[u]==color[v])
            printf("yes\n");
        else
            printf("no\n");
    }
    return 0;
}

C++: #include <iostream>
#include <vector>
#include <algorithm>
#include <queue>
#include <stack>
#include <stdio.h>
#include <cmath>
#include <set>
#include <map>
using namespace std;

vector<vector<int> >g;
int n,m,q;

vector<bool>visited;
bool dfs(int s,int t){
    if(s==t)return true;
    visited[s]=true;
    for(int i=0;i<g[s].size();i++){
        if(!visited[g[s][i]]){
            if(dfs(g[s][i],t)){
                return true;
            }
        }
    }
    return false;
}
int main() {
    while(1){}
    cin>>n>>m;
    g.resize(n);
    
    for(int i=0;i<m;i++){
        int s,t;
        cin>>s>>t;
        g[s].push_back(t);
        g[t].push_back(s);
    }
    cin>>q;
    for(int i=0;i<q;i++){
        visited.resize(n,false);
        int s,t;
        cin>>s>>t;
        cout<<(dfs(s,t)?"yes":"no")<<endl;
    }
    return 0;
}