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 <map>
#include <set>
#include <list>
#include <cmath>
#include <deque>
#include <queue>
#include <stack>
#include <cstdio>
#include <string>
#include <vector>
#include <complex>
#include <cstdlib>
#include <cstring>
#include <numeric>
#include <utility>
#include <iostream>
#include <algorithm>
#include <functional>
#include <unordered_map>
#include <unordered_set>

using namespace std;

#define ll long long
#define ull unsigned long long
#define INF (1 << 30)
#define LLINF (1LL << 62)
#define MOD7 1000000007
#define MOD9 1000000009
#define eps 1e-9
#define P pair<int, int>
#define pii pair<int, int>
#define pll pair<ll, ll>
#define pul pair<ull, ull>
#define all(a) (a).begin(), (a).end()
#define FOR(i,a,b) for (int i=(a);i<(b);++i)
#define RFOR(i,a,b) for (int i=(b)-1;i>=(a);--i)
#define REP(i,n) for (int i=0;i<(n);++i)
#define RREP(i,n) for (int i=(n)-1;i>=0;--i)

void YES() { cout << "YES" << endl; }
void NO() { cout << "NO" << endl; }
void Yes() { cout << "Yes" << endl; }
void No() { cout << "No" << endl; }
void yes() { cout << "yes" << endl; }
void no() { cout << "no" << endl; }

template<class T>
class UnDirectedGraph {
public:
	class Edge {
	public:
		int to;
		T cost;

		Edge(int t, T c) : to(t), cost(c) {}

		bool operator<(const Edge& right) const {
			return cost == right.cost ? to < right.to : cost < right.cost;
		}
	};

	vector<vector<Edge>> G;
	int V;

	UnDirectedGraph(int n = 0) {
		resize(n);
	}

	void resize(int n) {
		V = n;
		G.resize(n);
	}

	void addEdge(int from, int to, T cost = 0) {
		int v = max(from, to);
		if (v >= V)
			resize(v + 1);
		G[from].push_back(Edge(to, cost));
		G[to].push_back(Edge(from, cost));
	}

	vector<int> visit;
	int col = 0;
	void dfs(int s) {
		//vector<int> visit(V);
		visit.resize(V);
		col++;
		calcDfs(visit, s);
	}

	void calcDfs(vector<int>& visit, int s) {
		if (visit[s])
			return;

		//visit[s] = 1;
		visit[s] = col;
		for (int i = 0; i < G[s].size(); ++i) {
			calcDfs(visit, G[s][i].to);
		}
	}

	vector<int> bfs(int s) {
		vector<int> dist(V);
		queue<int> que;

		fill(dist.begin(), dist.end(), -1);
		dist[s] = 0;
		que.push(s);
		while (!que.empty()) {
			int from = que.front();
			que.pop();
			for (int i = 0; i < G[from].size(); ++i) {
				int to = G[from][i].to;
				if (dist[to] != -1)
					continue;
				dist[to] = dist[from] + 1;
				que.push(to);
			}
		}

		return dist;
	}
};

int main() {
	UnDirectedGraph<int> g;
	int n, m, from, to;
	cin >> n >> m;
	while (m--) {
		cin >> from >> to;
		g.addEdge(from, to);
	}
	
	for (int i = 0; i < n; ++i)
		g.dfs(i);

	int q;
	cin >> q;
	while (q--) {
		cin >> from >> to;
		if (g.visit[from] == g.visit[to])
			yes();
		else
			no();
	}

	return 0;
}

C++: #include<iostream>
#include<vector>
using namespace std;
static const int MAX = 100000;
static const int NIL = -1;

int n;
vector<int> G[MAX]; // List of vectors
int color[MAX];

void dfs_visit(int v, int id) {
    color[v] = id;
    for (int i=0; i<G[v].size(); i++) {
        int u = G[v][i];
        if (color[u]==NIL) {
            dfs_visit(u, id);
        }
    }
}

void dfs() {
    for (int v=0; v<n; v++) {
        color[v] = NIL;
    }
    int id = 0; // Cluster id
    for (int v=0; v<n; v++) {
        if (color[v]==NIL) {
            // If not clustered yet
            dfs_visit(v, 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);
    }

    dfs();

    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++: #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++: #include<iostream>
#include<vector>
#inlcude<stack>
using namespace std;
#define MAX 100000
#define NIL -1

int n;
vector<int> G[MAX];
int color[MAX];


void dfs(int r, int c) {
  stack<int> S;
  int i, v, u;
  S.push(r);
  color[r]=c;
  while (! S.empty()) {
    u=S.top();
    S.pop();
    for (i=0; i<G[u].size(); i++) {
        v=G[u][i];
      if (color[v]==NIL) {
        color[v]=c;
        S.push(v);
      }
    }
  }
}



void assignColor() {
  int i, u;
  int id=1;
  for (i=0; i<n; i++) color[i]=NIL;
  for (u=0; u<n; u++) {
    if (color[u]==NIL)
      dfs(u, id++);
  }
}

int main() {

  int  m, i, j, v, s, t, q;

  cin >> n >> m;

  for (i=0; i<m; i++) {
    cin >> s >> t;
    G[s].push_back(t);
    G[t].push_back(s);
  }

  assignColor();

  cin >> q;

  for (i=0; i<q; i++) {
    cin >> s >> t;
    if (color[s]== color[t]) {
      printf("yes\n");
    } else {
      printf("no\n");
    }
  }

  return 0;

}