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>
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++){
    int v=g[u][i];
    if(color[v]==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 <cstdlib>
#include <vector>

using namespace std;

struct UnionFind
{
    vector<int> data;
    UnionFind() {}
 
    // initialize
    UnionFind(int size) : data(size, -1) {}
 
    void unite_set(int x, int y){
        x = root(x);
        y = root(y);
        bool is_unite = (x != y);
        
        if (is_unite){
            if (data[y] < data[x]){
                swap(x, y);
            }
            data[x] += data[y];
            data[y] = x;
        }
    }
 
    bool is_same(int x, int y){
        return root(x) == root(y);
    }
 
    int root(int x){
        return (data[x] < 0) ? x : data[x] = root(data[x]);
    }
 
    int size(int x){
        return -data[root(x)];
    }
};

int main()
{
    int n, q, m;
    int a, b;
    cin >> n;
    struct UnionFind d(n);
    vector<int> data;
    data.resize(n);
    
    for (int i=0; i<n; ++i){
        data[i] = i;
    }
    // unite
    cin >> m;
    for (int i=0; i<m; ++i){
        cin >> a >> b;
        d.unite_set(a, b);
    }
    // find
    cin >> q;
    for (int i=0; i<q; ++i){
        cin >> a >> b;
        if(d.is_same(a, b)){
            cout << "yes" << endl;
        }
        else {
            cout << "no" << endl;
        }
    }
}

C++: #include <iostream>
#include <stack>
#include <vector>
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 v, int id)
{
    stack<int> S;
    S.push(v);
    color[v] = id;
    int u;
    while (!S.empty()) {
        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] = id;
                S.push(v);
            }
        }
    }
}

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

int main(int argc, char const *argv[])
{
    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<list>
#include<iterator>
#define MAX 100000
using namespace std;
char ans[MAX + 1];

class person
{
public:
	int id;
	bool Isdone = false;
	list <person*> lis;
	bool serch(person* Per)
	{
		if (ans[Per->id] == 1)return true;
		else if (ans[Per->id] == -1)return false;
		Isdone = true;
		for(list<person*>::iterator temp=lis.begin();temp!=lis.end();++temp)
		{
			if ((*temp) == Per)
			{
				ans[(*temp)->id] = 1;
				return true;
			}
			else if ((*temp)->Isdone == true)continue;
			else
			{
				if ((*temp)->serch(Per))
				{
					ans[(*temp)->id] = 1;
					return true;
				}
			}
		}
		ans[Per->id] = -1;
		return false;
	}
};

person P[MAX + 1];

int main()
{
	int n, m;
	cin >> n >> m;
	for (int i = 0; i < n; i++)P[i].id = i;
	for (int i = 0; i < m; i++)
	{
		int p1, p2;
		scanf("%d%d",&p1, &p2);
		P[p1].lis.push_back(&P[p2]);
		P[p2].lis.push_back(&P[p1]);
	}			 
	int q;
	cin >> q;
	for (int i = 0; i < q; i++)
	{
		int p1, p2;
		scanf("%d%d", &p1, &p2);
		if (P[p1].serch(&P[p2]))
		{
			cout << "yes" << endl;
		}
		else
		{
			cout << "no" << endl;
		}
		for (int l = 0; l < n; l++)P[l].Isdone = false;
		for (int l = 0; l < n; l++)ans[l] = 0;
	}
	return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define repr(i, n) for (int i = (int)(n); i >= 0; i--)
#define REP(i, m, n) for (int i = (int)(m); i <= (int)(n); i++)
#define REPR(i, m, n) for (int i = (int)(m); i >= (int)(n); i--)
#define all(v) v.begin(), v.end()
typedef long long ll;

using Graph = vector<vector<int> >;

vector<int> seen;

void dfs(const Graph &G, int v, int c){
  seen[v] = c;

  for(int nv : G[v]){
    if(seen[nv] != -1) continue;
    dfs(G, nv, c);
  }
}

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

  Graph G(n);
  rep(i, m){
    int a, b; cin >> a >> b;
    G[a].push_back(b);
    G[b].push_back(a);
  }

  seen.assign(n, -1);
  int c = 0;
  rep(i, n){
    if(seen[i] == -1){
      dfs(G, i, c);
      c++;
    }
  }

  int q; cin >> q;
  rep(i, q){
    int s, t; cin >> s >> t;
    cout << (seen[s] == seen[t] ? "yes" : "no") << endl;
  }

  return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;

int main()
{
	int n, m;
	cin >> n >> m;
	vector<vector<int>> g( n );
	for( int i = 0; i < m; i++ ) {
		int s, t;
		cin >> s >> t;
		g[s].push_back( t );
		g[t].push_back( s );
	}
	vector<int> color( n );
	int col = 0;
	for( int i = 0; i < n; i++ ) {
		if( color[i] > 0 ) continue;
		col++;
		color[i] = col;
		queue<int> que;
		que.push( i );
		while( !que.empty() ) {
			int u = que.front();
			que.pop();
			for( auto v : g[u] ) {
				if( color[v] > 0 ) continue;
				que.push( v );
				color[v] = col;
			}
		}
	}

	int q;
	cin >> q;
	for( int i = 0; i < q; i++ ) {
		int s, t;
		cin >> s >> t;
		string ans = "no";
		if( color[s] == color[t] ) ans = "yes";
		cout << ans << endl;
	}
}