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>

struct node {
	int parent;
	std::vector<int> relation;
};

void dfs(std::vector<node> &n) {
	std::stack<int> s;
	std::vector<int> d(n.size()), dd(n.size());
	for (size_t i = 0; i < n.size(); i++) {
		if (dd[i] == 1)continue;
		n[i].parent = -1;
		std::fill(d.begin(), d.end(), 0);
		s.push(i);
		while (s.size() != 0) {
			int num = s.top(); s.pop();
			d[num] = 1;
			dd[num] = 1;
			for (std::vector<int>::iterator it = n[num].relation.begin(); it != n[num].relation.end(); it++) {
				if (d[*it] == 1) {
					n[num].relation.erase(it);
					it--;
				} else {
					s.push(*it);
					d[*it] = 1;
					n[*it].parent = num;
					std::vector<int>::iterator itt= std::find(n[*it].relation.begin(), n[*it].relation.end(), num);
					if (itt != n[*it].relation.end()) {
						n[*it].relation.erase(itt);
					}
				}
			}
		}
	}
}

void makeparent(std::vector<node> &n) {
	std::queue<int> q;
	for (size_t i = 0; i < n.size(); i++) {
		n[i].parent = -1;
	}
	for (size_t i = 0; i < n.size(); i++) {
		q.push(i);
		int parent = i;
		while (q.size() != 0) {
			parent = q.front(); q.pop();
			for (size_t j = 0; j < n[parent].relation.size(); j++) {
				q.push(n[parent].relation[j]);
				n[n[parent].relation[j]].parent = parent;
			}
		}
	}
}

bool isconnect(std::vector<node> n, int from, int to) {
	std::stack<int> s;
	while (n[from].parent != -1) {
		from = n[from].parent;
		if (from == to) return true;
	}
	s.push(from);
	while(s.size() != 0) {
		from = s.top(); s.pop();
		for (std::vector<int>::iterator it = n[from].relation.begin(); it != n[from].relation.end(); it++) {
			if (to == *it) return true;
			s.push(*it);
		}
	}
	return false;
}

int main() {
	int people, relation;

	std::cin >> people >> relation;

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

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

	dfs(n);
	//makeparent(n);

	/*
	for (size_t i = 0; i < n.size(); i++) {
	std::cout << i << " " << n[i].parent << std::endl;
	}
	*/
	/*
	for (size_t i = 0; i < n.size(); i++) {
		for (size_t j = 0; j < n[i].relation.size(); j++) {
			std::cout << i << n[i].relation[j] << std::endl;
		}
	}
	//*/

	int question;
	std::cin >> question;

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

	return 0;
}

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

int find(vector<vector<int>> &frnd, vector<int> &state, int u){
    for (int i = 0; i < frnd[u].size(); i++){
        if (state[frnd[u][i]] == 0){
            return frnd[u][i];
        }
    }
    return -1;
}

int main(){
    int n, m;
    cin >> n >> m;
    vector<vector<int>> frnd(n);
    int s, t;
    for (int i = 0; i < m; i++){
        cin >> s >> t;
        frnd[s].push_back(t);
        frnd[t].push_back(s);
    }
    vector<int> color(n, 0);
    int cl = 1;
    for (int i = 0; i < n; i++){
        stack<int> s;
        //vector<int> state(n, 0);
        if (color[i] == 0){
            s.push(i);
            color[i] = cl;
            //state[i]++;
            while(!s.empty()){
                int v = s.top();

                s.pop();
                for (int j = 0; j < frnd[v].size(); j++){
                    int u = frnd[v][j];
                    if (color[u] == 0){
                        color[u] = cl;
                        s.push(u);
                    }
                }
                /*int nxt = find(frnd, state, v);
                if (nxt != -1){
                    s.push(nxt);
                    color[nxt] = cl;
                    state[nxt]++;
                }
                else{
                    state[v]++;
                    s.pop();
                }*/
            }
        }
        cl++;
    }
    int q;
    cin >> q;
    int a, b;
    for (int i = 0; i < q; i++){
        cin >> a >> b;
        if (color[a] == color[b])
            cout << "yes" << endl;
        else
            cout << "no" << endl;
    }
    return 0;
}

C++: #ifdef LOCAL_BUILD
#include "pch.h"
#define DLOG(msg) cout << "#" << __LINE__ << ":" << msg << endl;
#define DLOG_V(var) \
  cout << "#" << __LINE__ << ":" << #var << " : " << var << endl;
#else
#include <bits/stdc++.h>
#define DLOG(msg)
#define DLOG_V(var)
#endif
using namespace std;

struct Solver {
  explicit Solver(int n)
      : connected_(n, vector<bool>(n, false)) {}
  void register_friend(int s, int t) {
    connected_[s][t] = connected_[t][s] = true;
  }
  bool can_reach(int s, int t) {
    if (s == t)
      return true;
    vector<bool> done(connected_.size(), false);
    queue<int> q;
    q.push(s);
    while (!q.empty()) {
      auto v = q.front();
      q.pop();
      if (v == t)
        return true;
      if (done[v])
        continue;
      done[v] = true;
      auto& con = connected_[v];
      for (size_t i = 0; i < con.size(); ++i) {
        if (con[i]) {
          auto v2 = static_cast<int>(i);
          if (v2 == t)
            return true;
          if (!done[v2]) {
            q.push(v2);
          }
        }
      }
    }
    return false;
  }

 private:
  vector<vector<bool>> connected_;
};

int main() {
  int n, m;
  cin >> n >> m;
  Solver sol(n);
  for (int i = 0; i < m; ++i) {
    int s, t;
    cin >> s >> t;
    sol.register_friend(s, t);
  }
  int q;
  cin >> q;
  for (int i = 0; i < q; ++i) {
    int s, t;
    cin >> s >> t;
    auto ok = sol.can_reach(s, t);
    cout << (ok ? "yes" : "no") << endl;
  }
}