Go: package main

import (
	"fmt"
)

var n, m int
var g [][]int
var color []int

func main() {
	fmt.Scan(&n, &m)
	g = make([][]int, n) // SNSの関係を表すグラフ
	for i := 0; i < m; i++ {
		var a, b int
		fmt.Scan(&a, &b)
		g[a] = append(g[a], b) // 無向重みなしグラフ
		g[b] = append(g[b], a)
	}
	color = make([]int, n) // 各ノードにつける色
	for i := 0; i < len(color); i++ {
		color[i] = -1 // 初期化しておく
	}
	type query struct {
		s, t int
	}
	var Q int
	fmt.Scan(&Q)
	qs := make([]query, Q)
	for i := 0; i < Q; i++ {
		fmt.Scan(&qs[i].s, &qs[i].t)
	}
	// fmt.Println("n", n, "m", m, "g", g, "Q", Q, "qs", qs)
	// 全ての頂点からdfsする
	id := 1
	for u := 0; u < n; u++ {
		if color[u] == -1 {
			dfs(u, id)
			id++
		}
	}
	// fmt.Println("color", color)
	for i := 0; i < Q; i++ {
		if color[qs[i].s] == color[qs[i].t] {
			fmt.Println("yes")
		} else {
			fmt.Println("no")
		}
	}
}

func dfs(v, c int) {
	// fmt.Printf("dfs(v %d, c %d) en:\n", v, c)
	color[v] = c
	for _, vv := range g[v] {
		// fmt.Printf("  v %d, g[v] %v, vv %d\n", v, g[v], vv)
		if color[vv] == -1 {
			dfs(vv, c)
		}
	}
}

C++: #include <cassert>
#include <algorithm>
#include <iostream>
#include <vector>
#include <string>

class TVertex
{
    int FIndex;
    int FCommuniyId;
    std::vector<TVertex*> FList;

public:
    TVertex(int Index)
     : FIndex(Index), FCommuniyId(0)
    {}
    void AddPath(TVertex* Vertex)
    {
        FList.push_back(Vertex);
    }
    void SetCommunity(int CommunityId)
    {
        if(FCommuniyId) return;
        FCommuniyId = CommunityId;
        for(std::vector<TVertex*>::iterator It = FList.begin(); It != FList.end(); ++It){
            TVertex* V = *It;
            V->SetCommunity(CommunityId);
        }
    }
    int CommunityId()const
    {
        return FCommuniyId;
    }
};
class TGraph
{
    std::vector<TVertex*> FTable;
    void Init(int n)
    {
        assert(n > 0);
        FTable.resize(n);
        for(int i = 0; i < n; ++i){
            FTable[i] = new TVertex(i);
        }
    }
public:
    TGraph(int n)
    {
        Init(n);
    }
    
    void SetupVertex(std::istream& in)
    {
        int Index;
        in >> Index;
        assert(Index >= 1);
        assert(Index <= FTable.size());
        int k;
        in >> k;
        for(int i = 0; i < k; ++i){
            int v;
            in >> v;
            FTable[Index]->AddPath(FTable[v]);
        }
    }
    void MakeFriend(int s, int t)
    {
        assert(s < FTable.size());
        assert(t < FTable.size());
        FTable[s]->AddPath(FTable[t]);
        FTable[t]->AddPath(FTable[s]);
    }
    void SetCommunity()
    {
        for(std::size_t i = 0; i < FTable.size(); ++i){
            FTable[i]->SetCommunity(i+1);
        }
    }
    bool IsSameCommunity(int s, int t)const
    {
        assert(s < FTable.size());
        assert(t < FTable.size());
        return FTable[s]->CommunityId() == FTable[t]->CommunityId();
    }
};

int main()
{
    int n, m;
    std::cin >> n >> m;
    TGraph Graph(n);
    for(int i = 0; i < m; ++i){
        int s, t;
        std::cin >> s >> t;
        Graph.MakeFriend(s, t);
    }
    int q;
    std::cin >> q;
    Graph.SetCommunity();
    for(int i = 0; i < q; ++i){
        int s, t;
        std::cin >> s >> t;
        std::cout << (Graph.IsSameCommunity(s, t) ? "yes" : "no") << std::endl;
    }
    return 0;
    
}

C++: #include <cstdio>
#include <algorithm>
#include <vector>
#include <set>
using namespace std;

int main()
{
  int n, m, v1, v2;
  scanf("%d%d", &n, &m);
  vector<set<int>> v;
  set<int> s;

  for (int i = 0; i < m; i++)
  {
    scanf("%d%d", &v1, &v2);
    set<int> st = {v1, v2};
    vector<set<int>> ve;

    for (int j = 0; j < v.size(); j++)
    {
      s = v[j];
      if (s.find(v1) != s.end() || s.find(v2) != s.end())
        st.insert(s.begin(), s.end());
      else
        ve.push_back(s);
    }

    ve.push_back(st);
    v = ve;
  }

  int q;
  scanf("%d", &q);

  for (int i = 0; i < q; i++)
  {
    scanf("%d%d", &v1, &v2);
    bool found = false;
    for (int j = 0; j < v.size() && !found; j++)
    {
      s = v[j];
      if (s.find(v1) != s.end() && s.find(v2) != s.end())
        found = true;
    }
    if (found)
      printf("yes\n");
    else
      printf("no\n");
  }

  return 0;
}

C++: #include <iostream>
#include<stdio.h>
#include<vector>
#include<stack>
#define MAX 100000

using namespace std;
vector<int> G[MAX];
int color[MAX], n;

void dfs_visit(int r, int id) { //???????????°
    stack<int> S;
    S.push(r);
    color[r] = id;
    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] = id;
                S.push(v);
            }
        }
    }
}

void assignColor() { // ?????????????????°
    int id = 1;
    for(int i = 0; i < n; i++) {
        color[i] = -1;
    }
    for(int i = 0; i < n; i++) {
        if(color[i] == -1)
            dfs_visit(i, id++);
    }
}

int main()
{
    int m;
    scanf("%d%d", &n, &m);

    int s, t;
    for(int i = 0; i < m; i++) {
        scanf("%d%d", &s, &t);
        G[s].push_back(t);
        G[t].push_back(s);
    }

    assignColor();
    int q;
    scanf("%d", &q);
    for(int i = 0; i < q; i++) {
        scanf("%d%d", &s, &t);
        if(color[s] == color[t])
            printf("yes\n");
        else
            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;
}

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

bool is_friend(const vector<vector<int> >& u, int s, int t, vector<int>& searched){
    if (s == t) return true;
    for (int i = 0; i < u[s].size(); i++) {
        if (!searched[u[s][i]]) {
            searched[u[s][i]] = 1;
            return is_friend(u, u[s][i], t, searched);
        }
    }
    return false;
}
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;
        vector<int> searched(n, 0);
        if (is_friend(u, s, t, searched)) {
            cout << "yes" << endl;
        } else {
            cout << "no"  << endl;
        }
    }
    return 0;
}