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 <cstdio>
#include <vector>
using namespace std;
#define MAX_N 100000
int n;

bool can_connect(int s,int t,const vector< vector<int> > &g) {
  bool visited[n];
  queue<int> que;
  for(int i=0;i<n;i++) visited[i] = false;
  while(!que.empty()) que.pop();

  visited[s] = true;
  que.push(s);
  while(!que.empty()) {
    int v = que.front(); que.pop();
    for(int i=0;i<g[v].size();i++) {
      if(!visited[g[v][i]]) {
        if(g[v][i] == t) return true;
        visited[g[v][i]] = true;
        que.push(g[v][i]);
      }
    }
  }
  return false;
}

int main() {
  int m,s,t,q;
  scanf("%d %d\n",&n,&m);
  vector< vector<int> > g(n);
  for(int i=0;i<m;i++) {
    scanf("%d %d",&s,&t);
    g[s].push_back(t);
    g[t].push_back(s);
  }
  /*
  for(int i=0;i<g.size();i++) {
    printf("%d : ",i);
    for(int j=0;j<g[i].size();j++) {
      printf("%d ",g[i][j]);
    }
    printf("\n");
  }
  */

  scanf("%d",&q);
  for(int i=0;i<q;i++) {
    scanf("%d %d",&s,&t);
    //printf("%d,%d: ",s,t);
    if(can_connect(s,t,g)){
      printf("yes\n");
    }
    else {
      printf("no\n");
    }
  }
}

C++: #include <cstdio>
#include <algorithm>
#include <unordered_map>
#include <vector>
int main(){
    int n;
    int m;
    std::scanf("%d", &n);
    std::scanf("%d", &m);

    std::unordered_map<int, int> map;
    for(int i = 0; i < m; ++i){
        int s;
        int t;
        std::scanf("%d", &s);
        std::scanf("%d", &t);
        if(map.find(s) == map.end() && map.find(t) == map.end()){
            int small = std::min(s, t);
            int large = std::max(s, t);
            map.insert(std::make_pair(small, small));
            map.insert(std::make_pair(large, small));
        } else if(map.find(s) == map.end()){
            int key = map[t];
            while(key != map[key]){
                key = map[key];
            }
            if(key <= s){
                map.insert(std::make_pair(s, key));
            } else {
                map[key] = s;
                map.insert(std::make_pair(s, s));
            }
        } else if(map.find(t) == map.end()){
            int key = map[s];
            while(key != map[key]){
                key = map[key];
            }
            if(key <= t){
                map.insert(std::make_pair(t, key));
            } else {
                map[key] = t;
                map.insert(std::make_pair(t, t));
            }
        } else {
            int keys = map[s];
            while(keys != map[keys]){
                keys = map[keys];
            }
            int keyt = map[t];
            while(keyt != map[keyt]){
                keyt = map[keyt];
            }
            int small = std::min(keys, keyt);
            int large = std::max(keys, keyt);
            map[large] = small;
        }
    }

    int q;
    std::scanf("%d", &q);
    for(int i = 0; i < q; ++i){
        int s;
        int t;
        std::scanf("%d", &s);
        std::scanf("%d", &t);
        if(map.find(s) == map.end() || map.find(t) == map.end()){
            printf("no\n");
            continue;
        }
        int keys = map[s];
        while(keys != map[keys]){
            keys = map[keys];
        }
        int keyt = map[t];
        while(keyt != map[keyt]){
            keyt = map[keyt];
        }
        if(keys == keyt){
            std::printf("yes\n");
        } else {
            std::printf("no\n");
        }
    }

    return 0;
}

C++: #include <iostream>
#include <vector>
#include <stack>
using namespace std;
static const int MAX = 100000;
static const int NIL = -1;

int n;
vector<int> G[MAX];
int color[MAX];

void dfs(int r, int c) {
  stack<int> S;
  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] == NIL) {
        color[v] = c;
        S.push(v);
      }
    }
  }
}

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

int main() {
  int s, t, m, q;

  cin >> n >> m;

  for (int i = 0; i < m; i++) {
    cin >> s >> t;
    G[s].push_back(t);
    G[t].push_back(s);
  }

  assignColor();

  cin >> q;

  for (int i = 0; i < q; i++) {
    cin >> s >> t;
    if (color[s] == color[t]) {
      cout << "yes" << endl;
    } else {
      cout << "no" << endl;
    }
  }

  return 0;
}