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 <cstdio>
#include <cstring>
#include <stack>
#include <vector>
#include <queue>
using namespace std;

const int Max = 100005;
int color[Max], nt[Max];
vector<int> G[Max];
int n;
/*
int next(int u)
{
    nt[u]++;
    if(nt[u]<G[u].size())
        return G[u][nt[u]];
    else
        return -1;
}
*/
void dfs(int s, int t)
{

     //memset(nt, -1, sizeof(nt));
     stack<int> S;
     S.push(s);
     color[s] = t;
     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]==-1)
             {
                 color[v] = t;
                 S.push(v);
             }
         }
     }
     /*
     while(!S.empty())
     {
         int u = S.top();
         int v = next(u);
         if(v!=-1)
         {
             if(v==t) return 1;
             if(color[v]==-1)
             {
                 color[v] = 0;
                 S.push(v);
             }
         }
         else
         {
             S.pop();
         }
     }
     */
     //return 0;
}

void assignColor()
{
    int id = 1;
    memset(color, -1, sizeof(color));
    for(int i=0; i<n; i++)
    {
        if(color[i]==-1)
            dfs(i, id++);
    }
}

int main()
{
    int m, q, s, t;
    scanf("%d %d", &n, &m);
    for(int i=0; i<m; i++)
    {
        scanf("%d %d", &s, &t);
        G[s].push_back(t);//邻接表
        G[t].push_back(s);
    }
    scanf("%d", &q);
    assignColor();
    for(int i=0; i<q; i++)
    {
        scanf("%d %d", &s, &t);
        if(color[s]==color[t])
            printf("yes\n");
        else
            printf("no\n");
        /*
        if(dfs(s, t))
            cout << "yes" << endl;
        else
            cout << "no" << endl;
            */
    }

    return 0;
}

C++: #include <stdio.h>
#include <stdint.h>
#include <iostream>
#include <fstream>
#include <cassert>
#include <climits>

#include <cstring>
#include <queue>
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


const int kInvalid = -1;
const int kMaxUsers = 100 * 1000;

int g_group[kMaxUsers] = { [0 ... kMaxUsers-1] = kInvalid }; // 400 KiB

// 100 K * sizeof(vector<int>) + 100 K * sizeof(int)
// = 2.4 MiB + 100 K * sizeof(int)
vector<int> g_list[kMaxUsers];

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


void doGrouptize()
{
  // debug info
  DPRINTF("size of g_group[] %lu Byte\n", sizeof(g_group));
  DPRINTF("size of g_list[]  %lu Byte\n", sizeof(g_list));

  // init
  int groupId = 0;

  //memset(g_group, kInvalid, kMaxUsers);
  for (int i = 0; i < kMaxUsers; i++)
    g_group[i] = kInvalid;

  for (int i = 0; i < kMaxUsers; i++)
  {
    if (g_group[i] != kInvalid)
      continue;

    g_group[i] = groupId;

    // grouptize connected users in BFS order
    queue<int> que;
    que.push(i);

    while (!que.empty())
    {
      const int current = que.front();
      que.pop();

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

        if (g_group[next] != kInvalid)
          continue;

        g_group[next] = groupId;
        que.push(next);
      }
    }

    groupId++;
  }
}


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();

  doGrouptize();

  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);

    if (g_group[s] == g_group[t])
      cout << "yes" << endl;
    else
      cout << "no" << endl;
  }

  return 0;

}

C++: #include <bits/stdc++.h>
using namespace std;
int main(){
  int n, m;
  cin >> n >> m;
  vector<set<int>> E(n);
  for (int i = 0; i < m; i++){
    int s, t;
    cin >> s >> t;
    E[s].insert(t);
    E[t].insert(s);
  }
  vector<int> G(n, -1);
  for (int i = 0; i < n; i++){
    if (G[i] == -1){
      queue<int> Q;
      Q.push(i);
      G[i] = i;
      while (!Q.empty()){
        int v = Q.front();
        Q.pop();
        for (int w : E[v]){
          if (G[w] == -1){
            G[w] = G[v];
            Q.push(w);
          }
        }
      }
    }
  }
  int q;
  cin >> q;
  for (int i = 0; i < q; i++){
    int s, t;
    cin >> s >> t;
    if (G[s] == G[t]){
      cout << "yes" << endl;
    } else {
      cout << "no" << endl;
    }
  }
}