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<int> g[100000];
int c[100000],n;


void dfs(int r,int a){
  stack<int> s;
  s.push(r);
  c[r]=a;
  while(!s.empty()){
    int u=s.top();
    s.pop();
    for(int i=0;i<g[u].size();i++){
      int v=g[u][i];
      if(c[v]==-1){
	c[v]=a;
	s.push(v);
      }
    }
  }
}

void color(){
  int id=1;
  for(int i=0;i<n;i++) c[i]=-1;
  for(int i=0;i<n;i++){
    if(c[i]==-1) dfs(i,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);
  }
  color();
  cin>>q;
  for(int i=0;i<q;i++){
    cin>>s>>t;
    if(c[s]==c[t]) cout<<"yes"<<endl;
    else cout<<"no"<<endl;
  }
  return 0;
}

C++: #include <iostream> using namespace std; const int N = 100000; int A[N][N] = {{0}}; int n; bool dfs(int s, int t) {     if (A[s][t] == 1) {         return true;     }     for (int i = 0; i < n; ++i) {         if (i != t && A[s][i] != 0) {             if (dfs(i, t)) {                 return true;             }         }     }     return false; } int main() {     int m;     cin >> n >> m;     for (int i = 0; i < m; ++i) {         int s, t;         cin >> s >> t;         A[s][t] = 1;     }     int q;     cin >> q;     for (int j = 0; j < q; ++j) {         int x, y;         cin >> x >> y;         if (dfs(x, y)) {             cout << "yes";         } else {             cout << "no";         }         cout << endl;     }     return 0; }

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

int uni[100000];

int oya(int a){
if (uni[a]<0){
	return a;
}
else{
	uni[a]=oya(uni[a]);
	return oya(uni[a]);
}
}

bool ren(int a,int b){
	a=oya(a);
	b=oya(b);
	if(a==b){
	return	false;
	}
	else{
		
	return true;	
	}
}

int main() {

int n,m;
cin>>n>>m;
for(int y=0;y<n;y++){
	uni[y]=-1;
}

for(int r=0;r<m;r++){
int s,g;
cin>>s>>g;
if(ren(s,g)){
	s=oya(s);
	uni[s]=g;
	}
}
int q;
cin>>q;
for(int rr=0;rr<q;rr++){
	int s,g;
	cin>>s>>g;
	if(ren(s,g)==true){
		cout<<"NO"<<endl;
	}
	else{
		cout<<"YES"<<endl;
	}
}
	return 0;
}

C++: #include <iostream>
#include <vector>
#define eol '\n'
using namespace std;

vector<int> visit;

bool bfs(int s, int t, vector<vector<int>> list) {
  if (s == t) {
    return true;
  }
  
  // visit flag
  visit[s] = 1;
  
  for (int i = 0; i < list[s].size(); i++) {
    int next = list[s][i];
    if (visit[next] == 0) {
      if (bfs(next, t, list)) {
        return true;
      }
    }
  }
  return false;
}

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

  int n,m,s,t,q;
  cin >> n >> m;

  visit.resize(n,0);
  vector<vector<int>> list(n, {});
  
  for (int i = 0; i < m; i++) {
    cin >> s >> t;
    list[s].push_back(t);
    list[t].push_back(s);
  }

  cin >> q;
  for (int i = 0; i < q; i++) {
    cin >> s >> t;

    if (bfs(s,t,list)) {
      cout << "yes" << eol;
    } else {
      cout << "no" << eol;
    }
  }
  return 0;
}

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

vector<int>q[100010];
int color[100010],n;

void dfs(int r ,int id){
	color[r]=id;
	stack<int>s;
	s.push(r);
	while(!s.empty()){
		int u=s.top();s.pop();
		for(int i=0;i<q[u].size();i++){
			int v=q[u][i];
			if(color[v]==-1){
				color[v]=id;
				s.push(v);
			}
		}
	}
}
void paint(){
	int id=1;
	for(int i=0;i<n;i++){
		if(color[i]==-1){
			dfs(i,id);
			id++;
		}
	}
}

int main() {
	int m;
	cin>>n>>m;
	for(int i=0;i<m;i++){
		int s,t;
		cin>>s>>t;
		q[s].push_back(t);
		q[t].push_back(s);
	}
	for(int i=0;i<n;i++){
		color[i]=-1;
	}
	paint();
	int k;
	cin>>k;
	for(int i=0;i<k;i++){
		int s,t;
		cin>>s>>t;
		cout<< (color[s]==color[t]? "yes":"no");
		cout<<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;
    int i, u, v;
    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 id = 1;
    int i, u;
    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 s, t, m, q, i;
 
    cin >> n >> m;
 
    for ( i = 0; i < m; i++ ) {
        cin >> s >> t;
        G[s].push_back(t);
        G[t].push_back(s);
    }
 
    assignColor();
 
    cin >> s;
 
    for ( 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 <queue>
using namespace std;

vector<int> list[100000];
int n,m;

bool bfs(int s, int t){
  bool used[m];
  for(int i=0;i<m;i++) used[i]=false;
  queue<int> Q;
  Q.push(s); used[s]=true;
  while(!Q.empty()){
    int k=Q.front(); Q.pop();
    if(k==t){return true;}
    for(int i=0;i<list[k].size();i++){
      int temp=list[k][i];
      if(!used[temp]){
        Q.push(temp); used[temp]=true;
      }
    }
  }
  return false;
}
int main(){
  cin>>n>>m;
  for(int i=0;i<m;i++){
    int u,v; cin>>u>>v;
    list[u].push_back(v);
    list[v].push_back(u);
  }
  int q; cin>>q;
  for(int i=0;i<q;i++){
    int s,t; cin>>s>>t;
    if(bfs(s, t)){
      cout<<"yes"<<"\n";
    }else{
      cout<<"no"<<"\n";
    }
  }
}