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

#include <cstring>
#include <queue>
#include <map>
#include <utility>
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

/*
   Memory limitaion; 64 MiB
   # of users: 2 <= n <= 100 K
   # of relations: 0 <= m <= 100 K

   Data size:
    1) for relations (maybe hash table):
      100K * 4Byte * 2 (bi-direction) = 800K

    2) for visited flag:
      100K * 1Byte = 100K
*/

const int kMaxUsers = 100 * 1000;

bool g_isVisited[kMaxUsers]; // 100 KiB
vector<int> g_list[kMaxUsers];


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


string isReachable(int num, int start, int end)
{

  // init
  memset(g_isVisited, 0, num);

  queue<int> que;
  que.push(start);

  while (!que.empty())
  {
    int id = que.front();

    que.pop();
    g_isVisited[id] = true;

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

      if (next == end)
        return string("yes");

      if (!g_isVisited[next])
        que.push(next);
    }
  }

  return string("no");
}


void printAllRelations()
{
#if DEBUG
  for (auto& m: g_relations)
    DPRINTF("%d %d\n", m.first, m.second);
#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();

  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);

    cout << isReachable(n, s, t) << endl;
  }

  return 0;

}

C++: #include<bits/stdc++.h>
#define ll long long
#define rep(i, n) for(int i=0;i<(n);++i)
#define per(i, n) for(int i=(n)-1;i>=0;--i)
#define repa(i, n) for(int i=1;i<(n);++i)
#define foreach(i, n) for(auto &i:(n))
#define pii pair<int, int>
#define pll pair<long long, long long>
#define all(x) (x).begin(), (x).end()
#define bit(x) (1ll << (x))
const ll MOD = (ll)1e9+7;
const int INF = (ll)1e9+7;
const ll INFLL = (ll)1e18;
using namespace std;
template<class t>
using vvector = vector<vector<t>>;
template<class t>
using vvvector = vector<vector<vector<t>>>;
template<class t>
using priority_queuer = priority_queue<t, vector<t>, greater<t>>;
template<class t, class u> bool chmax(t &a, u b){if(a<b){a=b;return true;}return false;}
template<class t, class u> bool chmin(t &a, u b){if(a>b){a=b;return true;}return false;}

ll modpow(ll x, ll b){
	ll res = 1;
	while(b){
		if(b&1)res = res * x % MOD;
		x = x * x % MOD;
		b>>=1;
	}
	return res;
}

ll modinv(ll x){
	return modpow(x, MOD-2);
}

bool was_output = false;
template<class t>
void output(t a){
	if(was_output)cout << " ";
	cout << a;
	was_output = true;
}
void outendl(){
	was_output = false;
	cout << endl;
}

class union_find{
public:
	int n;
	vector<int> parent;
	union_find(int _n):n(_n){
		parent.resize(n);
		rep(i, n){
			parent[i]=i;
		}
	}

	int get_parent(int x){
		if(parent[x]==x)return x;
		return parent[x] = get_parent(parent[x]);
	}

	bool is_same(int a, int b){
		int ap = get_parent(a);
		int bp = get_parent(b);
		return ap == bp;
	}

	void merge(int a, int b){
		int ap = get_parent(a);
		int bp = get_parent(b);
		if(ap==bp)return;
		parent[bp] = ap;
	}

};

int main(){
	int n;
	int m;
	cin >> n >> m;
	union_find uni(n);
	rep(i, m){
		int a, b;
		cin >> a >> b;
		uni.merge(a, b);
	}
	int q;
	cin >> q;
	rep(i, q){
		int a, b;
		cin >> a >> b;
		cout << (uni.is_same(a, b)?"yes":"no") << endl;
	}
	return 0;
}

C++: #include <iostream>

using namespace std;

const int N = 100000;
int A[N][N];
int n;

bool dfs(int s, int t) {
    if (A[s][t] == 1) {
        return true;
    }
    for (int i = 0; i < n; ++i) {
        if (i != t && A[s][i] != 0) {
            if (dfs(i, t)) {
                return true;
            }
        }
    }
    return false;
}

int main() {
    int m;
    cin >> n >> m;
    for (int i = 0; i < m; ++i) {
        int s, t;
        cin >> s >> t;
        A[s][t] = 1;
    }

    int q;
    cin >> q;
    for (int j = 0; j < q; ++j) {
        int x, y;
        cin >> x >> y;

        if (dfs(x, y)) {
            cout << "yes";
        } else {
            cout << "no";
        }
        cout << endl;
    }

    return 0;
}

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

vector<int>cc;
void dfs(vector<vector<int>> &g,int v,int id){
    cc[v]=id;
    for(auto e:g[v]){
        if(cc[e]==-1){
            dfs(g,e,id);
        }
    }
}

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);
    }
    int q;
    cin>>q;
    vector<int> s(q),t(q);
    for(int i=0;i<q;++i){
        cin>>s.at(i)>>t.at(i);
    }
    cc.assign(n,-1);
    int id=0;
    for(int i=0;i<n;++i){
        if(cc[i]==-1){
            dfs(g,i,id);
            id++;
        }
    }
    for(int i=0;i<q;++i){
        if(cc[s[i]]==cc[t[i]])cout<<"yes"<<endl;
        else cout<<"no"<<endl;
    }
    return 0;
}