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 <bits/stdc++.h>

using namespace std;

vector<vector<int>> makelist(int n, int m) {
	vector<vector<int>> Adjlist(n);
	int a, b;

	for(int i = 0; i < m; i++) {
		cin >> a >> b;
		Adjlist[a].push_back(b);
		Adjlist[b].push_back(a);
	}

	return Adjlist;
}

bool dfs_run(int a, int b, vector<vector<int>>& adj, vector<bool>& checker) {
	while(!adj[a].empty()) {
		int tmp;
		tmp = adj[a].back();
		adj[a].pop_back();	
		if(tmp == b) {
			return true;
		}else{
			return dfs_run(tmp, b, adj, checker);
		}
	}
	return false;
}

bool dfs(int n, int a, int b, vector<vector<int>>& adj) {
	vector<bool> checker(n, true);
	
	checker[a] = false;
	if(dfs_run(a, b, adj, checker)) {
		return true;
	}
	return false;
}

int main(void)
{
	int n, m, q;

	cin >> n >> m;
	vector<vector<int>> Adjlist = makelist(n, m);

	cin >> q;
	for(int i = 0; i < q; i++) {
		int a, b;
		cin >> a >> b;

		if(dfs(n, a, b, Adjlist)) {
			cout << "yes" << endl;
		}else{
			cout << "no" << endl;
		}
	}

	return 0;
}

C++: #include <iostream>
#include <vector>
#include <algorithm>
#include <numeric>
#include <queue>
#include <stack>
#include <map>
#define FOR(i,b,e) for(int i=b;i<e;i++)
#define REP(i,n) FOR(i,0,n)
using namespace std;

const int N = 100000;
int n, m, q;
int M[N][N];
bool V[N];

int bfs(int u, int v) {
  int s;
  queue<int> q;
  REP(i, n) V[i] = false;

  s = u;
  q.push(s);
  V[s] = true;

  while (!q.empty() && !V[v]) {
    s = q.front(); q.pop();
    REP(i, n) {
      if (M[s][i] == 1 && !V[i]) {
        q.push(i);
        V[i] = true;
      }
    }
  }

  return V[v];
}

int main() {
	cin.tie(0);
 	ios::sync_with_stdio(false);

  REP(i, n) {
    REP(j, n) {
      M[i][j] = 0;
    }
  }

  cin >> n >> m;
  REP(i, m) {
    int u, v;
    cin >> u >> v;
    M[u][v] = M[v][u] = 1;
  }

  cin >> q;
  REP(i, q) {
    int u, v;
    string answer;
    cin >> u >> v;
    answer = bfs(u, v) ? "yes" : "no";
    cout << answer << endl;
  }
}

C++: using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace _20180319_AOJ
{
    class ALDS1_11D
    {
        static void Main(string[] args)
        {
            for (;;)
            {
                string line = Console.ReadLine();
                if (string.IsNullOrEmpty(line))
                {
                    break;
                }

                int[] n_m = line.Split(' ').Select(int.Parse).ToArray();
                int n = n_m[0];
                int m = n_m[1];
                int[,] matrix = new int[n, n];

                for (int i = 0; i < m; i++)
                {
                    int[] s_t = Console.ReadLine().Split(' ').Select(int.Parse).ToArray();
                    matrix[s_t[0], s_t[1]] = 1;
                    matrix[s_t[1], s_t[0]] = 1;
                }

                int[] group = new int[n];
                int gn = 0;
                for (int i = 0; i < n; i++)
                {
                    if (group[i] == 0)
                    {
                        gn++;
                        group[i] = gn;
                    }

                    gn = group[i];
                    for (int t = 0; t < n; t++)
                    {
                        if (matrix[i, t] == 1)
                        {
                            int tn = group[t];
                            if (tn > 0)
                            {
                                Replace(group, tn, gn);
                            }
                            else
                            {
                                group[t] = gn;
                            }
                        }
                    }
                }

                int q = int.Parse(Console.ReadLine());
                for (int i = 0; i < q; i++)
                {
                    int[] s_t = Console.ReadLine().Split(' ').Select(int.Parse).ToArray();
                    if (group[s_t[0]] == group[s_t[1]])
                    {
                        Console.WriteLine("yes");
                    }
                    else
                    {
                        Console.WriteLine("no");
                    }
                }
            }
        }

        static void Replace<T>(T[] arr, T source, T dest)
        {
            for (int i = 0; i < arr.Length; i++)
            {
                if (arr[i].Equals(source))
                {
                    arr[i] = dest;
                }
            }
        }
    }
}

C++: #include<iostream>
#include<algorithm>
#include<queue>
using namespace std;
#define MAX 10000

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;
}