Go: // AOJ ALDS1_11_D: http://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=ALDS1_11_D

package main

import (
	"bufio"
	"container/list"
	"fmt"
	"os"
	"strconv"
)

var nocolor = -1

type graph struct {
	graph  [][]int
	colors []int
}

func nextInt(sc *bufio.Scanner) int {
	sc.Scan()
	v, _ := strconv.Atoi(sc.Text())
	return v
}

func newGraph(nodeNum int) *graph {
	colors := make([]int, nodeNum)
	g := make([][]int, nodeNum)
	for i := 0; i < nodeNum; i++ {
		g[i] = []int{}
		colors[i] = nocolor
	}

	return &graph{g, colors}
}

func constructGraph(sc *bufio.Scanner) *graph {
	usersNum := nextInt(sc)
	linkNum := nextInt(sc)

	g := newGraph(usersNum)
	for i := 0; i < linkNum; i++ {
		u := nextInt(sc) // node
		l := nextInt(sc) // link node
		// edge each other
		g.graph[u] = append(g.graph[u], l)
		g.graph[l] = append(g.graph[l], u)
	}
	return g
}

func (g *graph) dfs(r, color int) {
	g.colors[r] = color

	stack := list.New()
	stack.PushFront(r)

	for stack.Len() > 0 {
		v := stack.Remove(stack.Front()).(int)
		for _, u := range g.graph[v] {
			if g.colors[u] == nocolor {
				g.colors[u] = color
				stack.PushFront(u)
			}
		}
	}
}

func (g *graph) assignColor() {
	color := 1
	for u := 0; u < len(g.graph); u++ {
		if g.colors[u] == nocolor {
			g.dfs(u, color)
			color++
		}
	}
}

func (g *graph) isLink(s, t int) bool {
	return g.colors[s] == g.colors[t]
}

func main() {
	sc := bufio.NewScanner(os.Stdin)
	sc.Split(bufio.ScanWords)

	// construct graph and friendly colors
	g := constructGraph(sc)
	g.assignColor()

	// judge link
	q := nextInt(sc)
	for i := 0; i < q; i++ {
		s := nextInt(sc)
		t := nextInt(sc)

		if g.isLink(s, t) {
			fmt.Println("yes")
		} else {
			fmt.Println("no")
		}
	}
}

C++: #include <iostream>
#include <vector>

using namespace std;

bool dfs(int s, int t, vector<vector<int>> sns, bool* visit) {
  if (s == t) {
    return true;
  }
  if (visit[s]) {
    return false;
  }

  visit[s] = true;

  int len = sns[s].size();
  for (int i = 0; i < len; i++) {
    int next = sns[s][i];
    if (dfs(next, t, sns, visit)) {
      return true;
    }
  }
  return false;
}

int main() {
  int n,m,q,s,t;
  cin >> n >> m;

  vector<vector<int>> sns(n);
  
  for (int i = 0; i < m; i++) {
    cin >> s >> t;

    sns[s].push_back(t);
    sns[t].push_back(s);
  }

  cin >> q;
  for (int i = 0; i < q; i++) {
    cin >> s >> t;
    bool* visit = new bool[n];
    if (dfs(s, t, sns, visit)) {
      cout << "yes" << endl;
    } else {
      cout << "no" << endl;
    }
  }
  return 0;
}

C++: #include <bits/stdc++.h>
#define be(v) (v).begin(),(v).end()
#define pb(q) push_back(q)
typedef long long ll;
using namespace std;
const ll mod=1000000007;
#define doublecout(a) cout<<fixed<<setprecision(10)<<a<<endl;
ll uni[100005];
ll root(ll u){
    if(uni[u]<0)return u;
    return uni[u]=root(uni[u]);
}

void connect(int a,int b){
    a=root(a);
    b=root(b);
    if(a==b)return;
    if(uni[a]<uni[b]){
        a^=b;
        b^=a;
        a^=b;
    }
    uni[a]+=uni[b];
    uni[b]=a;
    return;
}
ll size(ll u){
    return -uni[root(u)];
}
int main() {
    cin.tie(0);
    cout.tie(0);
    ios::sync_with_stdio(false);
    memset(uni,-1,sizeof(uni));
    int n,m,q,a,b;
    cin>>n>>m;
    for(int i=0;i<m;i++){
    	cin>>a>>b;
    	connect(a,b);
    }
    vector<string> v;
    cin>>q;
    for(int i=0;i<q;i++){
    	cin>>a>>b;
    	if(root(a)==root(b))v.pb("yes");
    	else v.pb("no");
    }
    for(int i=0;i<q;i++){
    	cout << v[i] <<endl;
    }
    return 0;
}

C++: // ALDS1_11_D.cpp
// Graph I - Connected Components

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

void bfs(vector<vector<int>> &sns, vector<int> &arrived, int x, int y)
{
    if (arrived[y] != 0) return;
    queue<int> mq;
    mq.push(x);
    while(!mq.empty()) {
        int curr = mq.front();
        mq.pop();
        arrived[curr] = 2;
        for (int i = 0; i < sns[curr].size(); i++) {
            int next = sns[curr][i];
            if (arrived[next] == 0) {
                mq.push(next);
                arrived[next] = 1;
            }
        }
    }
}

int main()
{
//    freopen("ALDS1_11_D_in22.txt", "r", stdin);
//    freopen("out22.txt", "w", stdout);
    int m, n, q;
    cin >> n >> m;
    vector<vector<int> > sns(n, vector<int>());
    int x, y;
    for (int i = 0; i < m; i++) {
        cin >> x >> y;
        sns[x].push_back(y);
        sns[y].push_back(x);
    }

    cin >> q;
    while (q--) {
        vector<int> arrived(n);
        cin >> x >> y;
        bfs(sns, arrived, x, y);
        if (arrived[y]) cout << "yes" << endl;
        else cout << "no" << endl;
    }
    return 0;
}

C++: // ALDS1_11_D.cpp
// Graph I - Connected Components

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

bool isFriend(vector<vector<int>> &sns, vector<int> &arrived, int x, int y)
{
    for (int i = 0; i < sns[x].size(); i++) {
        if (sns[x][i] == y) return true;
    }

    bool res = false;
    arrived[x] = 1;
    for (int i = 0; i < sns[x].size(); i++) {
        int next = sns[x][i];
        if (!arrived[next]) {
            bool tmp = isFriend(sns, arrived, next, y);
            if (tmp) {
                res = true;
                break;
            }
        }
    }
    return res;
}

int main()
{
//    freopen("ALDS1_11_D_in20.txt", "r", stdin);
//    freopen("out20.txt", "w", stdout);
    int m, n, q;
    cin >> n >> m;
    vector<vector<int> > sns(n, vector<int>());
    int x, y;
    for (int i = 0; i < m; i++) {
        cin >> x >> y;
        sns[x].push_back(y);
        sns[y].push_back(x);
    }

    cin >> q;
    while (q--) {
        vector<int> arrived(n);
        cin >> x >> y;
        bool res = isFriend(sns, arrived, x, y);
        if (res) cout << "yes" << endl;
        else cout << "no" << endl;
    }
    return 0;
}