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>
#include<stack>
#include<queue>
#include<algorithm>
#include<ctime>

void dfs(std::vector<std::vector<int> > &n) {
	int question;
	std::cin >> question;

	for (int i = 0; i < question; i++) {
		int from, to;
		std::cin >> from >> to;
		std::stack<int> s;
		std::vector<int> went(n.size(), 0);

		s.push(from);
		bool flag = false;
		while (s.size() != 0 && flag == false){
			int num = s.top(); s.pop();
			for (size_t j = 0; j < n[num].size(); j++) {
				if (n[num][j] == to) {
					flag = true;
					break;
				}
				if (went[n[num][j]] == 0) {
					s.push(n[num][j]);
					went[n[num][j]] = 1;
				}
			}
		}
		if (flag) {
			std::cout << "yes" << std::endl;
		} else {
			std::cout << "no" << std::endl;
		}
	}
}

int main() {
	int people, relation;

	std::cin >> people >> relation;

	std::vector<std::vector<int> > n(people);

	for (int i = 0; i < relation; i++) {
		int from, to;
		std::cin >> from >> to;
		n[from].push_back(to);
		n[to].push_back(from);
	}

	dfs(n);

	return 0;
}

C++: #include<iostream>
#include<algorithm>
#include<queue>
using namespace std;
#define MAX 50000

struct Node{
	int group;
	queue<int> fr;
};

int N, M, Q, group_num = 0;
Node person[MAX];

void isFriend(int a, int b){
	if(person[a].group < 0 && person[b].group < 0){
		int p = a, next, m;
		queue<int> move;
		group_num++;
		person[p].group = group_num;
		//cout << a << " is group " << person[a].group << endl;
		move.push(p);

		while(!move.empty()){
			p = move.front();
			if(person[p].fr.empty()){
				move.pop();
			} else {
				m = person[p].fr.size();
				while(m--){
					next = person[p].fr.front();
					person[p].fr.pop();
					if(person[next].group < 0){
						person[next].group = group_num;
						//cout << next << " is group " << person[next].group << endl;
						move.push(next);
					}
				}
				move.pop();
			}
		}

		if(person[a].group == person[b].group){
			cout << "yes" << endl;
		} else {
			cout << "no" << endl;
		}
	} else if(person[a].group == person[b].group){
		cout << "yes" << endl;
	} else {
		cout << "no" << endl;
	}
}

int main(){
	cin >> N >> M;
	for(int i = 0; i < N; i++)
		person[i].group = -1;
	while(M--){
		int a, b;
		cin >> a >> b;
		person[a].fr.push(b);
		person[b].fr.push(a);
	}
	cin >> Q;
	while(Q--){
		int a, b;
		cin >> a >> b;
		isFriend(a, b);
	}
	return 0;
}

C++: #include<iostream>
using namespace std;
#include<stdio.h>

typedef struct box { int val; box *prev, *next; } box_t;

box_t* init()
{
    box_t* head = new box_t;
    head->val = -1;
    head->next = NULL;
    head->prev = NULL;
    return head;
}

box_t* make_tail(box_t* head)
{
    box_t* tail = new box_t;
    tail->val = -1;
    head->next = tail;
    tail->next = NULL;
    tail->prev = head;
    return tail;
}

bool isEmpty(box_t* head, box_t* tail)
{
    return (head->next == tail ? true : false);
}

box_t* f_insert(box_t* head, int data)
{
    box_t* box = new box_t;
    box->val = data;
    box->next = head->next;
    box->prev = head;
    head->next->prev = box;
    head->next = box;
    return box;
}

int b_pop(box_t* tail)
{
    if(!(tail->prev->prev)) return -1;
    box_t* box = tail->prev;
    int val;
    val = tail->prev->val;
    tail->prev = box->prev;
    box->prev->next = tail;
    delete box;
    return val;
}

void clear(box_t* head, box_t* tail)
{
    box_t* box1 = head->next;
    box_t* box2;
    if(box1 == tail) return;
    while(box1 != tail){
        box2 = box1->next;
        delete box1;
        box1 = box2;
    };
    head->next = tail;
    tail->prev = head;
    return;
}

int main()
{
    int G[10000][10000];
    int dist[10000];

    box_t* head = init();
    box_t* tail = make_tail(head);

    int n, i, j, e, q, v1, v2;
    scanf("%d", &n);
    for(i = 0; i < n; i++){
        for(j = 0; j < n; j++){ G[i][j] = 0; } }
    int m;
    scanf("%d", &m);
    for(int k = 0; k < m; k++){
        scanf("%d %d", &i, &j);
        G[i][0]++; G[j][0]++;
        G[i][G[i][0]] = j;
        G[j][G[j][0]] = i;
    }
    scanf("%d", &q);
    int self, friends;
    bool is_connected;

    for(int k = 0; k < q; k++){
        for(j = 0; j < n; j++) dist[j] = -1;

        scanf("%d %d", &self, &friends);
        f_insert(head, self);
        dist[self] = 0;
        is_connected = false;

        while(1){
            v1 = b_pop(tail);
            e = G[v1][0];
            for(i = 1; i <= e; i++){
                v2 = G[v1][i];
                if(dist[v2] < 0){
                    dist[v2] = dist[v1] + 1;
                    f_insert(head, v2);
                }
            }
            if(dist[friends] > 0){
                is_connected = true;
                break;
            }
            if(isEmpty(head, tail)) break;
        };
        if(is_connected){ printf("yes\n"); }else{ printf("no\n"); }
        if(!isEmpty(head, tail)) clear(head, tail);
    }

    return 0;
}

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

void color(std::vector<std::vector<int> > n) {
	std::vector<int> color(n.size(), 0);
	std::stack<int> s;

	int count = 1;
	for (size_t i = 0; i < n.size(); i++) {
		if (color[i] != 0) continue;
		s.push(i);
		while (s.size() != 0) {
			int num = s.top(); s.pop();
			for (std::vector<int>::iterator it = n[num].begin(); it != n[num].end(); it++) {
				if (color[*it] == 0) {
					s.push(*it);
					color[*it] = count;
				}
			}
		}
		count++;
	}

	int question;
	std::cin >> question;
	for (int i = 0; i < question; i++) {
		int from, to;
		std::cin >> from >> to;
		if (color[from] == color[to]) {
			std::cout << "yes" << std::endl;
		} else {
			std::cout << "no" << std::endl;
		}
	}

}

int main() {
	int people, relation;

	std::cin >> people >> relation;

	std::vector<std::vector<int> > n(people);

	for (int i = 0; i < relation; i++) {
		int from, to;
		std::cin >> from >> to;
		n[from].push_back(to);
		n[to].push_back(from);
	}

	color(n);

	return 0;
}