Go: package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
)

func getScanner(fp *os.File) *bufio.Scanner {
	scanner := bufio.NewScanner(fp)
	scanner.Split(bufio.ScanWords)
	scanner.Buffer(make([]byte, 500001), 500000)
	return scanner
}

func getNextString(scanner *bufio.Scanner) string {
	scanner.Scan()
	return scanner.Text()
}

func getNextInt(scanner *bufio.Scanner) int {
	i, _ := strconv.Atoi(getNextString(scanner))
	return i
}

func main() {
	fp := os.Stdin
	if len(os.Args) > 1 {
		fp, _ = os.Open(os.Args[1])
	}

	scanner := getScanner(fp)
	writer := bufio.NewWriter(os.Stdout)

	n := getNextInt(scanner)
	m := getNextInt(scanner)
	nodes := make([]Node, n)

	for i := 0; i < n; i++ {
		nodes[i].list = make([]int, 0)
	}

	for i := 0; i < m; i++ {
		s := getNextInt(scanner)
		t := getNextInt(scanner)
		nodes[s].list = append(nodes[s].list, t)
		nodes[t].list = append(nodes[t].list, s)
	}

	c := 1
	for i := 0; i < n; i++ {
		if nodes[i].c == 0 {
			traverse(nodes, i, c)
			c++
		}
	}

	q := getNextInt(scanner)

	for i := 0; i < q; i++ {
		s := getNextInt(scanner)
		t := getNextInt(scanner)
		if nodes[s].c == nodes[t].c {
			fmt.Fprintln(writer, "yes")
			continue
		}
		fmt.Fprintln(writer, "no")
	}

	writer.Flush()
}

// Node ...
type Node struct {
	list []int
	c    int
}

func traverse(nodes []Node, r, c int) {
	nodes[r].c = c
	for _, i := range nodes[r].list {
		if nodes[i].c != 0 {
			continue
		}
		traverse(nodes, i, c)
	}
}

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 <stdio.h>
#include <stack>
#include <vector>

static const int MAX = 100000;
static const int NIL = -1;

static int n;
static std::vector<int> G[MAX];
static int color[MAX];

static 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);
            }
        }
    }
}

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

int main()
{
    int m, q;

    scanf("%d %d", &n, &m);

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

    assignColor();

    scanf("%d", &q);

    for (int i = 0; i < q; i++) {
        int s, t;
        scanf("%d %d", &s, &t);
        if (color[s] == color[t]) {
            printf("yes\n");
        } else {
            printf("no\n");
        }
    }

    return 0;
}

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

const int Max = 100005;
int color[Max], nt[Max];
vector<int> G[Max];
int n;
/*
int next(int u)
{
    nt[u]++;
    if(nt[u]<G[u].size())
        return G[u][nt[u]];
    else
        return -1;
}
*/
void dfs(int s, int t)
{

     //memset(nt, -1, sizeof(nt));
     stack<int> S;
     S.push(s);
     color[s] = t;
     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] = t;
                 S.push(v);
             }
         }
     }
     /*
     while(!S.empty())
     {
         int u = S.top();
         int v = next(u);
         if(v!=-1)
         {
             if(v==t) return 1;
             if(color[v]==-1)
             {
                 color[v] = 0;
                 S.push(v);
             }
         }
         else
         {
             S.pop();
         }
     }
     */
     //return 0;
}

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

int main()
{
    int m, q, s, t;
    scanf("%d %d", &n, &m);
    for(int i=0; i<m; i++)
    {
        scanf("%d %d", &s, &t);
        G[s].push_back(t);//邻接表
        G[t].push_back(s);
    }
    scanf("%d", &q);
    assignColor();
    for(int i=0; i<q; i++)
    {
        scanf("%d %d", &s, &t);
        if(color[s]==color[t])
            printf("yes\n");
        else
            printf("no\n");
        /*
        if(dfs(s, t))
            cout << "yes" << endl;
        else
            cout << "no" << endl;
            */
    }

    return 0;
}

C++: #include <stdio.h>
#include <stdint.h>
#include <iostream>
#include <fstream>
#include <cassert>
#include <climits>

#include <cstring>
#include <queue>
using namespace std;

#define DEBUG (0)

#if DEBUG
#define DPRINTF(...) do { \
    printf("D(L%d) %s: ", __LINE__, __func__); \
    printf(__VA_ARGS__); \
  } while (false)
#else
#define DPRINTF(...)
#endif // DEBUG


const int kInvalid = -1;
const int kMaxUsers = 100 * 1000;

int g_group[kMaxUsers] = { [0 ... kMaxUsers-1] = kInvalid }; // 400 KiB

// 100 K * sizeof(vector<int>) + 100 K * sizeof(int)
// = 2.4 MiB + 100 K * sizeof(int)
vector<int> g_list[kMaxUsers];

void setRelations(int start, int end)
{
  g_list[start].push_back(end);
  g_list[end].push_back(start);
}


void doGrouptize()
{
  // debug info
  DPRINTF("size of g_group[] %lu Byte\n", sizeof(g_group));
  DPRINTF("size of g_list[]  %lu Byte\n", sizeof(g_list));

  // init
  int groupId = 0;

  //memset(g_group, kInvalid, kMaxUsers);
  for (int i = 0; i < kMaxUsers; i++)
    g_group[i] = kInvalid;

  for (int i = 0; i < kMaxUsers; i++)
  {
    if (g_group[i] != kInvalid)
      continue;

    g_group[i] = groupId;

    // grouptize connected users in BFS order
    queue<int> que;
    que.push(i);

    while (!que.empty())
    {
      const int current = que.front();
      que.pop();

      for (int j = 0; j < g_list[current].size(); j++)
      {
        const int next = g_list[current][j];

        if (g_group[next] != kInvalid)
          continue;

        g_group[next] = groupId;
        que.push(next);
      }
    }

    groupId++;
  }
}


void printAllRelations()
{
#if DEBUG
  for (int i = 0; i < kMaxUsers; i++)
  {
    for (int j = 0; j < g_list[i].size(); j++)
    {
      if (j == 0)
        DPRINTF("%d", i);

      printf(" %d", g_list[i][j]);

      if (j == g_list[i].size() - 1)
        cout << endl;
    }
  }
#endif // DEBUG
}


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

  DPRINTF("number of users %d\n", n);
  DPRINTF("number of relations %d\n", m);

  for (int i = 0; i < m; i++)
  {
    int s, t;
    cin >> s >> t; cin.ignore();

    setRelations(s, t);
  }

  printAllRelations();

  doGrouptize();

  int q;
  cin >> q; cin.ignore();

  for (int i = 0; i < q; i++)
  {
    int s, t;
    cin >> s >> t; cin.ignore();

    DPRINTF("src %d  dst %d\n", s, t);

    if (g_group[s] == g_group[t])
      cout << "yes" << endl;
    else
      cout << "no" << endl;
  }

  return 0;

}