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<stdio.h>
#include<vector>
#include<string>
#include<iostream>
using namespace std;

vector<vector<int> >g;
int n,m,q,s,t;

vector<bool>visited;
vector<int>color;
void dfs(int s){
    for(int i=0;i<g[s].size();i++){
        if(color[g[s][i]]==-1){
            color[g[s][i]]=color[s];
            dfs(g[s][i]);
        }
    }
}
bool dfs2(int s,int t){
    if(s==t)return true;
    visited[s]=true;
    for(int i=0;i<g[s].size();i++){
        if(!visited[g[s][i]]){
            if(dfs2(g[s][i],t)){
                return true;
            }
        }
    }
    return false;
}
int main() {
    scanf("%d %d",&n,&m);
    g.resize(n);
    color.resize(n,-1);
    for(int i=0;i<m;i++){
        scanf("%d %d",&s,&t);
        g[s].push_back(t);
        g[t].push_back(s);
    }
    for(int i=0;i<n;i++){
        if(color[i]==-1){
            color[i]=i;
            dfs(i);
        }
    }
    scanf("%d",&q);
    for(int i=0;i<q;i++){
        scanf("%d %d",&s,&t);
        visited.resize(n,false);
        string s1=(dfs2(s,t)?"yes":"no");
        string s2=(color[s]==color[t]?"yes":"no");
        if(s1!=s2&&s1=="yes")return 1;
        cout<<s2<<endl;
    }
    return 0;
}

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());
	for (size_t i = 0; i < n.size(); i++) {
		std::fill(d.begin(), d.end(), 0);
		s.push(i);
		while (s.size() != 0) {
			int num = s.top(); s.pop();
			d[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;
					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 <bits/stdc++.h>
using namespace std;
const int N = 1000000;
const int CRL = -1;
int v[N]={0};
void ketugou(int k,int l,int n){
	int x,mn;
	x = max(v[k],v[l]);
	mn = min(v[k],v[l]);
	for(int i=0;i<n;i++){
		if(v[i]==x)v[i]=mn;
	}
}

int main(){
	ios::sync_with_stdio(false);
	cin.tie(0);
	int n,m,q;
	memset(v, CRL, sizeof(v));
	cin >> n >> m;
	for(int i=0;i<m;i++){
		int k,l;
		cin >> k >> l;
		if(v[k]==CRL && v[l]==CRL){
			v[k] = v[l] = min(k,l);
		}else if(v[k]!=CRL && v[l]!=CRL){
			ketugou(k,l,n);
		}else if(v[k]==CRL){
			v[k]=v[l];
		}else{
			v[l]=v[k];
		}
	}
	cin >> q;
	for(int i=0;i<q;i++){
		int k,l;
		cin >> k >> l;
		if(v[k]==CRL || v[l]==CRL) cout << "no" << endl;
		else if(v[k]==v[l])cout << "yes" << endl;
		else cout << "no" << endl;
	}
    return 0;
}

C++: #include <iostream>
#include<string>
#include<cmath>
#include<algorithm>
#include<cctype>
#include<queue>
#include<stack>
#include<stdio.h>
#include<vector>
#include<set>
#include<map>
#include<array>
#include<iomanip>
#define rep(i,n) for(int i=0;i<n;i++)
typedef int long long ll;

using namespace std;

static const int MAX=1000000;
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;
    rep(i,n)color[i]=nil;
    rep(i,n){
        if(color[i]==nil){
            dfs(i,id);
            id++;
        }
    }
    
    
}

int main(){
    int s,t,m,q;
    cin>>n>>m;
    rep(i,m){
        cin>>s>>t;
        g[s].push_back(t);
        g[t].push_back(s);
    }
    assignColor();
    
    cin>>q;
    
    rep(i,q){
        cin>>s>>t;
        if(color[s]==color[t]){
            cout<<"yes"<<endl;
        }else{
            cout<<"no"<<endl;
        }
    }
    rep(i,n){
        cout<<i<<" "<<color[i]<<endl;
    }
    

return 0;
}