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++: #ifdef LOCAL_BUILD
#include "pch.h"
#define DLOG(msg) cout << "#" << __LINE__ << ":" << msg << endl;
#define DLOG_V(var) \
  cout << "#" << __LINE__ << ":" << #var << " : " << var << endl;
#else
#include <bits/stdc++.h>
#define DLOG(msg)
#define DLOG_V(var)
#endif
using namespace std;

struct Solver {
  explicit Solver(int n)
      : connected_(n, vector<bool>(n, false)) {}
  void register_friend(int s, int t) {
    connected_[s][t] = connected_[t][s] = true;
  }
  bool can_reach(int s, int t) {
    if (s == t)
      return true;
    vector<bool> done(connected_.size(), false);
    queue<int> q;
    q.push(s);
    while (!q.empty()) {
      auto v = q.front();
      q.pop();
      if (v == t)
        return true;
      if (done[v])
        continue;
      done[v] = true;
      auto& con = connected_[v];
      for (size_t i = 0; i < con.size(); ++i) {
        if (con[i]) {
          auto v2 = static_cast<int>(i);
          if (v2 == t)
            return true;
          if (!done[v2]) {
            q.push(v2);
          }
        }
      }
    }
    return false;
  }

 private:
  vector<vector<bool>> connected_;
};

int main() {
  int n, m;
  cin >> n >> m;
  Solver sol(n);
  for (int i = 0; i < m; ++i) {
    int s, t;
    cin >> s >> t;
    sol.register_friend(s, t);
  }
  int q;
  cin >> q;
  for (int i = 0; i < q; ++i) {
    int s, t;
    cin >> s >> t;
    auto ok = sol.can_reach(s, t);
    cout << (ok ? "yes" : "no") << endl;
  }
}

C++: // 連結成分
#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
#define NIL -1

vector<int> G[100005];
int color[100005];
int n,col;

void dfs(int r, int c);
void assign_number();

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

    col = 0;
    assign_number();
    
    int q;
    cin >> q;
    
    for(int i = 0; i < q; i++){
        int s,t; cin >> s >> t;
        cout << (color[s] == color[t] ? "yes" : "no") << endl;
    }
    return 0;
}

void assign_number(){
    // 未訪問の頂点を選んで探索していく
    for(int i = 0; i < n; i++){
        color[i] = NIL;
    }

    for(int i = 0; i < n; i++){
        if(color[i] == NIL){
            dfs(i,col); col++;
        }
    }
}

void dfs(int r, int c){
    // 頂点rから探索しそのグループにcの番号をふる
    stack<int> s;
    s.push(r);
    color[r] = c;

    while(!s.empty()){
        int top = s.top();
        s.pop();
        int size = G[top].size();
        for(int i = 0; i < size; i++){
            int v = G[top][i];
            if(color[v] == NIL){
                // まだ探索されていない頂点
                color[v] = c;
                s.push(v);
            }
        }
    }

}

C++: #include <cstdio>
#include <vector>
using namespace std;
const int MAXN = 1e5 + 7;
int col = 1;
int idx[MAXN];
vector<int> G[MAXN];
void dfs(int cur) {
	idx[cur] = col;
	for (int i = 0; i < G[cur].size(); i++) {
		if (!idx[G[cur][i]]) dfs(G[cur][i]);
	}
}
int main() {
    int n, m;
    scanf("%d%d", &n, &m);
    while (m--) {
    	int s, t;
    	scanf("%d%d", &s, &t);
    	G[s].push_back(t);
    	G[t].push_back(s);
	}
	for (int i = 0; i < n; i++) {
		if (!idx[i]) {
			dfs(i);
			col++;
		}
	}
	int q;
	scanf("%d", &q);
	for (int i = 0; i < q; i++) {
		int s, t;
		scanf("%d%d", &s, &t);
		if (idx[s] == idx[t]) printf("yes\n");
		else printf("no\n");
	}
	return 0;
}

C++: #include<stack>
#include<queue>
#include<conio.h>
#include<string>
#include<vector>
#include<iostream>
#include<algorithm>
using namespace std;
queue<int> Q;
vector<int> G[100000];
bool flag = false;
int s, t;
int color[100000];

void a(){
	while (!Q.empty()){
		Q.pop();
		for (int i = 0; i < G[s].size(); i++){
			//_getch();
			//cout << G[s][i] << endl;
			if (G[s][i] == t){
				flag = true;
				while (!Q.empty()) Q.pop();
				return;
			}
			else if (color[i] == 0){
				Q.push(G[s][i]);
				color[i] = 1;
			}
		}
		if (!Q.empty())s = Q.front();
	}
}

int main(){
	int n, 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);
	}
	cin >> q; fill(color, color + 100000, 0);
	for (int i = 0; i < q; i++){
		cin >> s >> t;
		Q.push(s);
		a();
		if (flag) cout << "yes" << endl;
		else cout << "no" << endl;
		flag = false;
		fill(color, color + 100000, 0);
	}

	return 0;
}