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<cstdio>
#include<vector>
#define N 100000
using namespace std;

vector<int> G[N];
/*
??£??\??????????????????????????°???????????¨??????????????£????????????????????????DFS??§????????????

 */
int labels[N];

void dfs_init(){
    int i;
    for(i=0;i<N; i++)labels[i]=0;
}

/*
  ?????°?????¨??????????£?
  ??§???????????????????????????
  stack???????????°??§???????????????
 */
bool dfs(int u, int v){
    if (u == v) return true;
    labels[u] = 1;
    int i,x;
    for(i=0; i<G[u].size(); i++){
	x = G[u][i];
	dfs(x,v);
    }
    return false;
}

void dfs2(int u){
    labels[u]=1;
    int i;
    for(i=0; i<G[u].size(); i++){
	if (labels[G[u][i]]==0) dfs2(G[u][i]);
    }
}


int main(void) {
    int n,m,i,q, u,v;
    scanf("%d %d",&n,&m);
    for(i=0;i<m;i++){
	scanf("%d %d",&u,&v);
	G[u].push_back(v);
	G[v].push_back(u);
    }

    
    scanf("%d",&q);
    for(i=0;i<q;i++){
	scanf("%d %d", &u, &v);
	dfs_init();
	dfs2(u);
	if(labels[u]>0 && labels[u]==labels[v])printf("yes\n");
	else printf("no\n");
    }
    
    return 0;
}

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

using namespace std;

const int N = 100000;
int n;
int color[N];
vector<int> v[N];

bool isFriend(int s, int t){
  int color[N] = {0};
  queue<int> q;
  int x;
  q.push(s);
  while(!q.empty()){
    x = q.front(); q.pop();
    for(int i=0;i<n;i++){
      if(v[x][i] == 0) continue;
      if(color[i] == 0){
        if(i == t) return true;
        color[i] = 1;
        q.push(i);
      }
    }
  }
  return false;
}

int main(){
  int m;
  cin >> n >> m;
  int s, t;
  for(int i=0;i<m;i++){
    cin >> s >> t;
      v[s].push_back(t);
      v[t].push_back(s);
  }
  int q; cin >> q;
  for(int i=0;i<q;i++){
    cin >> s >> t;
    if(isFriend(s, t)) cout << "yes" << endl;
    else cout << "no" << endl;
  }
  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++: // http://hfuji.hatenablog.jp/entry/2015/12/09/175721

#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cassert>
#include <string>
#include <cstring>
#include <vector>
#include <list>
#include <map>
#include <set>
#include <stack>
#include <queue>
#include <algorithm>

using namespace std;

// ---------------------
// repetition
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n) FOR(i,0,n)

// debug
#define dump(x)  cerr << #x << " = " << (x) << endl;
#define debug(x) cerr << #x << " = " << (x) << " (L" << __LINE__ << ")" << " " << __FILE__ << endl;

// ---------------------
#define INF 922337203685477580
typedef long long ll;
vector<int> par;
vector<int> rnk; // ??±???

void init(int n) {
  par = vector<int>(n);
  rnk = vector<int>(n);
  REP(i, n) {
    par[i] = i;
    rnk[i] = 0;
  }
}
int root(int key) {
  if (par[key] == key) return key;
  return root(par[key]);
}
int same(int a, int b) {
  return root(a) == root(b);
}
void unite(int a, int b) {
  int x = root(a);
  int y = root(b);
  if (x == y) return;

  if (rnk[x] < rnk[y]) {
    par[x] = y;
  } else {
    par[y] = x;
    if (rnk[x] == rnk[y]) rnk[x]++;
  }
}

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

  init(n);

  REP(_, m) {
    int a, b;
    // cin >> a >> b;
    scanf("%d %d", &a, &b);
    unite(a, b);
  }
  int q;
  cin >> q;
  REP(_, q) {
    int a, b;
    // cin >> a >> b;
    scanf("%d %d", &a, &b);
    if (same(a, b)) {
      cout << "yes" << endl;
    } else {
      cout << "no" << endl;
    }
  }

  return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
#define MAX 1001001
#define NIL -1

/*
  疎なグラフ(連結成分が少ないグラフ)では、隣接行列を使うのはメモリefficientではない。
  ->隣接リストを用いて各ノードの関係を表す

  連結成分を求めるには、未訪問のノードを始点としてDFSを行う。
  この探索を行うごとに、各点に異なるIDを割り振ることで、どのグループに属するかを判定する。
*/

int n;
vector<int> G[MAX]; //頂点数がMAXまで格納できる隣接リスト(中身は配列)
int color[MAX]; //

void dfs_colorAssign(int start, int group_id) {
  stack<int> s;
  s.push(start);
  color[start] = group_id;

  while(!s.empty()) {
    int now = s.top(); s.pop();

    for (int i = 0; i < G[now].size(); i++) {
      int v = G[now][i];
      if (color[v] == NIL) {
        color[v] = group_id;
        s.push(v);
      }
    }
  }
}

void assign_color() {
  int id = 1;
  for (int i = 0; i < n; i++) color[i] = NIL; //color配列を初期化
  //未訪問の頂点はidがNILになる
  for (int i = 0; i < n; i++) {
    if (color[i] == NIL) dfs_colorAssign(i, id);
    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); // 互いの隣接リストに相手を追加
    G[t].push_back(s);
  }

  assign_color();

  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;
}