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<iostream>
#include<vector>
#include<stack>
using namespace std;

bool DFS(const vector<vector<int> >& G, int a, int b) {
    stack<pair<int, int> > S;
    vector<bool> is_visited(G.size(), false);

    int idx = 0;
    do {
        if (G[a][idx] == b) return true;
        if (!is_visited[G[a][idx]]) {
            is_visited[G[a][idx]] = true;
            S.push(make_pair(a, idx));
            a   = G[a][idx];
            idx = 0;
            continue;
        }
        if (idx >= G[a].size()) {
            a   = S.top().first;
            idx = S.top().second;
            S.pop();
        }
        ++idx;
    } while (!S.empty());
    return false;
}


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

    vector<vector<int> > G(n);

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

// for (int i = 0; i < n; i++) {
//     cout << i << " : ";
//     for (int j = 0; j < G[i].size(); j++){
//         cout << G[i][j] << " ";
//     }
//     cout << endl;
// }



    int q;
    cin >> q;
    for (int i = 0; i < q; i++) {
        int a, b;
        cin >> a >> b;
        if (DFS(G, a, b)) {
            cout << "yes" << endl;
        } else {
            cout << "no"  << endl;
        }
    }
    return 0;
}

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

const int MAX_N = 100000;

int G[MAX_N];
set<int> A[MAX_N];

void assign_gid(int user_num) {
  int gid;
  queue<int> Q;

  Q.push(0);
  G[0] = gid = 0;

  while (!Q.empty()) {
    int c = Q.front(); Q.pop();

    if (G[c] == -1) G[c] = ++gid;

    for (set<int>::iterator it = A[c].begin(); it != A[c].end(); it++) {
      if (G[*it] == -1) {
        G[*it] = gid;
        Q.push(*it);
      }
    }

    if (Q.empty()) {
      for (int i = 1; i < user_num; i++) {
        if (G[i] == -1) {
          Q.push(i);
          break;
        }
      }
    }
  }
}

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

  for (int i = 0; i < n; i++) G[i] = -1;

  for (int i = 0; i < m; i++) {
    int s, d; cin >> s >> d;
    A[s].insert(d);
    A[d].insert(s);
  }

  assign_gid(n);

  for (int i = 0; i < n; i++)
    cout << G[i] << ' ';
  cout << endl;

  int q; cin >> q;

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

  return 0;
}

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

void step(vector<vector<int> >& edges, vector<int>& node_class, int node, int cls)
{
    if(node_class[node] != -1) return;
    node_class[node] = cls;
    for(int e : edges[node])
    {
        if(node_class[e] == -1) step(edges, node_class, e, cls);
    }
}

void cls_init(vector<int>& node_class)
{
    for(vector<int>::size_type i=0;i<node_class.size();++i)
        node_class[i] = -1;
}

void dfs(vector<vector<int> >& edges, vector<int>& node_class)
{
    int cls = 0;
    cls_init(node_class);
    for(vector<int>::size_type i=0;i<edges.size();++i)
    {
        if(node_class[i] == -1) step(edges, node_class, i, cls++);
    }
}

int main()
{
    int n, m, s, t;
    cin >> n >> m;
    vector< vector<int> > edges(n);
    vector<int> node_class(n);
    for(int i=0;i<m;++i)
    {
        cin >> s >> t;
        edges[s].push_back(t);
        edges[t].push_back(s);
    }
    dfs(edges, node_class);
    int q;
    cin >> q;
    for(int i=0;i<q;++i)
    {
        cin >> s >> t;
        if(
            node_class[s] == node_class[t]
            && node_class[s] != -1
            && node_class[t] != -1
        ){ 
            cout << "yes";
        }
        else cout << "no";
        cout << endl;
    }
}

C++: #include<iostream>
#include<climits>
#include<vector>
#include<list>
#include<functional>
#include<algorithm>
#include<string>
#include<cmath>
#include<complex>
#include<set>
#include<map>
#include<stack>
#include<queue>
#include<tuple>
#define FOR(i,a,b) for(int i=(a);i<(b);i++)
#define REP(i,n) for(int i=0;i<(n);i++)
#define INF (int)1e9
#define INFL (ll)1e11
using namespace std;
typedef long long ll;
typedef pair<int, int> P;

string dfs(int s, int t, vector<vector<int>> v, vector<int> &used) {
	if (s == t) return "yes\n";
	used[s] = 1;
	int n = v.size();
	int k = v[s].size();
	REP(l, k) {
		int j = v[s][l];
		if (!used[j]) {
			return dfs(j, t, v, used);
		}
	}
	return "no\n";
}

int main() {
	cin.tie(0);
	ios::sync_with_stdio(false);

	int n, m;
	cin >> n >> m;
	vector<vector<int>> v(n);
	REP(i, m) {
		int s, t;
		cin >> s >> t;
		v[s].push_back(t);
		v[t].push_back(s);
	}
	int q;
	cin >> q;
	REP(i, q) {
		int s, t;
		cin >> s >> t;
		vector<int> used(n, 0);
		cout << dfs(s, t, v, used);
	}
	

	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;
            if (is_friend(u, u[s][i], t, searched)) return true;
        }
    }
    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);
        searched[s] = 1;
        if (is_friend(u, s, t, searched)) {
            cout << "yes" << endl;
        } else {
            cout << "no"  << endl;
        }
    }
    return 0;
}

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

int n;
std::vector<int> G[MAX];
int color[MAX];
void DFS(int,int);
void assignColor(void);

int main(){
  int s, t, m, q;
  std::cin >> n >> m;
  for(int i=0; i<m; i++){
    std::cin >> s >> t;
    G[s].push_back(t);
    G[t].push_back(s);
  }
  assignColor();
  std::cin >> q;
  for(int i=0; i<q; i++){
    std::cin >> s >> t;
    if(color[s] == color[t]) std::cout << "yes\n";
    else std::cout << "no\n";
  }
  return 0;
}
void assignColor(void){
  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++);
}
void DFS(int r, int c){
  std::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);
      }
    }
  }
}