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++: //16D8103013E Sen Shunkou
#include<iostream>
#include<queue>
int main(){
    //input
    int n,m;
    std::cin>>n;
    std::cin>>m;
    int A[n][n];
    bool visited[n];
    int judge[n];

    for(int i=0;i<n;i++){
        judge[i]=-1;
        visited[i]=false;
        A[i][0]=0;
        for(int j=1;j<n;j++){
            A[i][0]=0;
            A[i][j]=-1;
        }    
    }

    int cnt_i=0,cnt_j=0;
    for(int k=0;k<m;k++){
        int i,j;
        std::cin>>i;
        std::cin>>j;
        A[i][0]++; cnt_i=A[i][0];
        A[j][0]++; cnt_j=A[j][0];
        A[i][cnt_i]=j;
        A[j][cnt_j]=i;
    }

    /*for(int i=0;i<10;i++){
        for(int j=0;j<10;j++){
            std::cout<<" "<<A[i][j];
        }
        std::cout<<std::endl;
    }*/

    int n_2;
    std::cin>>n_2;
    for(int i=0;i<n_2;i++){
        int s,e;
        std::cin>>s;
        std::cin>>e;
        
        int cnt=0;
        for(int i=0;i<n;i++){
            judge[i]=-1;
            visited[i]=false;
        }
        
        std::queue<int> que;
        que.push(s);
        /*for(int i=0;i<10;i++){
            for(int j=0;j<10;j++){
                //std::cout<<" "<<A[i][j];
            }
            //std::cout<<std::endl;
        }*/
        while(!que.empty()){
            int q=que.size();
            for(int k=0;k<q;k++){
                //std::cout<<"quesize"<<que.size()<<"\n";
                int i=que.front();
                que.pop();
                visited[i]=true;
                judge[i]=cnt;
                //std::cout<<i<<"cnt"<<cnt<<"\n";
                //std::cout<<A[i][0]<<"\n";
                if(A[i][0]!=0){
                    for(int j=0;j<A[i][0];j++){
                        if(visited[A[i][j+1]]!=true){
                            visited[A[i][j+1]]=true;
                            que.push(A[i][j+1]);
                        }
                    }
                }
            }
            cnt++;
        }
        if(judge[e]==-1){
            std::cout<<"no\n";
        }else std::cout<<"yes\n";
    }

    return 0;
}

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

const int Max = 100000;
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){
                S.push(v);
                color[v] = c;
                // cout << color[v] << endl;
            }
        }
    }
}
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 m, u, v, q;
    cin >> n >> m;
    for(int i=0; i<m; i++){
        cin >> u >> v;
        G[u].push_back(v);
        G[v].push_back(u);
    }

    assignColor();
    
    cin >> q;
    for(int i=0; i<q; i++){
        cin >> u >> v;
        // cout << color[u] << color[v] << endl;
        if(color[u] == color[v]) cout << "yes\n";
        else cout << "no\n";
    }

    return 0;
}

C++: #include <stdio.h>
#include <stack>
#include <iostream>
using namespace std;
int G[10000][10000];
int vis[100000];
int n;

bool dfs(int s, int t)
{
    if (G[s][t] == 1 || G[t][s] == 1) return true;
    vis[s] = 1;
    for (int i = 0; i < n; i++)
    {
        if (G[s][i] == 1 && vis[i] == 0)
            {
                bool a = (dfs(i,t)||dfs(t,i)); 
                vis[s] = 0;
                return a;
            }

    } 
    vis[s] = 0;
    return false;
}

int main()
{
    //freopen("in3.txt","r",stdin);
    int m;
    scanf("%d %d",&n, &m);
    for (int i = 0; i < n; i++)
    {
        vis[i] = 0; 
    } 
    for (int i = 0; i < n; i++)
    {
        for (int j = 0; j < n; j++)
        {
            G[i][j] = 0; 
        }  
    } 
    for (int i = 0; i < m; i++)
    {
        int s, t;
        scanf("%d %d",&s,&t); 
        G[s][t] = 1;
        G[t][s] = 1;
    } 
    int q,s,t;
    scanf("%d",&q);
    for (int i = 0; i < q; i++)
    {
        scanf("%d %d",&s,&t);
        if(dfs(s,t)||dfs(t,s)) printf("yes\n");
        else printf("no\n");
    } 
    return 0;
}