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++: /*
* @Author: Samson
* @Date:   2018-05-07 21:12:41
* @Last Modified by:   Samson
* @Last Modified time: 2018-05-07 21:44:27
*/
//   @URL : 
#include<bits/stdc++.h>
#include<algorithm>
#include<vector>
#include<stack>
#include <cstdlib>
#define INF 0x3f3f3f3f
using namespace std;
typedef long long LL;
const int MAXN = 1e5+10;

int n,m,q,color[MAXN];
vector<int> v[MAXN];

void dfs(int x,int id)
{
	stack<int> s;
	int t;
	s.push(x);
	color[x] = id;
	while(s.size())
	{
		t = s.top();
		s.pop();
		for(int i = 0; i < v[t].size(); ++i)
		{
			int val = v[t][i];
			if(color[val] == -1)
			{
				color[val] = id;
				s.push(val);
			}
		}
	}
}

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

int main(void)
{
	ios::sync_with_stdio(false); 
	//cin.tie(0);
	cin>>n>>m;
	int s,e;
	for(int i = 0; i < m; ++i)
	{
		cin>>s>>e;
		v[s].push_back(e);
		v[e].push_back(s);
	}
	fun();
	cin>>q;
	while(q--)
	{
		cin>>s>>e;
		if(color[s] == color[e] && color[s]!=-1)
			cout<<"yes\n";
		else
			cout<<"no\n";
	}
	return 0;
}

C++: #include <iostream>
using namespace std;
 
int main() {
int d[100100];
int m,n,q;
cin>>n>>m;
for(int i=0;i<n;i++){
    d[i]=i;
}
for(int l=0;l<m;l++){
    int a,b;
    cin>>a>>b;
    int j ,h;
    j=a,h=b;
while(d[j]!=j ||d[h]!=h){
    j=d[j];
    h=d[h];
}

    d[j]=h;
    d[a]=h;
    d[b]=h;
d[d[a]]=h;
d[d[b]]=h;

}

cin>>q;
for(int y=0;y<q;y++){
    int a,b;
    cin>>a>>b;
        int j ,h;
    j=a,h=b;
while(d[j]!=j ||d[h]!=h){
    j=d[j];
    h=d[h];
}
    if(j==h){
        cout<<"yes"<<endl;
    }
    else{
        cout<<"no"<<endl;
    }
}
    return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;
#define rep(i, n) for(int i = 0; i < (int)(n); i++)

#define N 100000
#define NIL -1

int n, m;
vector<int> G[N];
int color[N];

queue<int> Q;

void bfs(int x, int c){
    if(color[x] != NIL) return;
    Q.push(x);
    color[x] = c;

    while(!Q.empty()){
        int u = Q.front(); Q.pop();
        for(int i = 0; i < G[u].size(); i++){
                int v = G[u][i];
                if(color[v] == NIL)
                Q.push(v);
                color[v] = c;
        }
    }
}

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


int main(){
    cin >> n >> m;

    rep(i,m){
        int a, b;
        cin >> a >> b;
        G[a].push_back(b);
        G[b].push_back(a);
    }

    assignColor();

    int q;
    cin >> q;
    rep(i,q){
        int x, y;
        cin >> x >> y;
        if(color[x] == color[y]) cout << "yes" << endl;
        else cout << "no" << endl;
    }

    return 0;

}

C++: #include<bits/stdc++.h>
using namespace std;
#define rep(i,n) for(int i = 0; i < (n) ; i++)
constexpr auto INF = 2147483647;
typedef long long ll;
constexpr int MAX = 100000;

vector<int> Group_list[MAX];
const int NIL = -1;
int Check_list[MAX];
int n;

void Checking(){
  //init
  rep(i,n) Check_list[i] = NIL;
  stack<int> stk;
  int check_cnt = 0;
  //solve
  rep(i,n){
    if(Check_list[i] == NIL) {
      stk.push(i);
      check_cnt++;
      Check_list[i] = check_cnt;
    }
    int now, next;
    while(!stk.empty()){
        now = stk.top();
        stk.pop();
        rep(j, Group_list[now].size()){
          next = Group_list[now][j];
          if(Check_list[next] == NIL) {
            Check_list[next] = check_cnt;
            stk.push(next);
          }
        }
    }
  }
}

int main(){
  int m,q;
  int from, to;
  cin >> n >> m;
  rep(i,m){
    cin >> from >> to;
    Group_list[from].push_back(to);
    Group_list[to].push_back(from);
  }

  Checking();

  cin >> q;
  rep(i,q){
    cin >> from >> to;
    if(Check_list[from] == Check_list[to] && Check_list[from] != NIL){
      cout << "yes" << endl;;
    }else{
      cout << "no" << endl;
    }
  }
  

  return 0;
}

C++: #include<iostream>
#include<vector>
using namespace std;

void set_color(const vector<vector<int> >& u, int p, vector<int>& color, int c) {
    if (color[p] != -1) return;
    color[p] = c;
    for (int i = 0; i < u[p].size(); i++) {
        set_color(u, u[p][i], color, c);
    }
}

bool is_friend(const vector<vector<int> >& u, int s, int t){
    static vector<int> color(u.size(), -1);
    static int c = 0;
    set_color(u, s, color, c++);
    return color[s] == color[t];
}

int main(){
    int n, m;
    cin >> n >> m;
    vector<vector<int> > u(n);
    for (int i = 0; i < m; i++) {
        int s, t;
        cin >> s >> t;
        u[s].push_back(t);
        u[t].push_back(s);
    }
    int q;
    cin >> q;
    for (int i = 0; i < q; i++) {
        int s, t;
        cin >> s >> t;
        if (is_friend(u, s, t)) {
            cout << "yes" << endl;
        } else {
            cout << "no"  << endl;
        }
    }
    return 0;
}