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 <stdlib.h>
#include <string.h>
#include <ctype.h>
#include <math.h>

//============================================================================
#define pred(x) ((x)-1)
#define succ(x) ((x)+1)
#define True    1
#define False   0
//#define max(x,y) (((x)<(y))?y:x)
//============================================================================
#define MAX_BUFFER  102
struct graph{
    long id;
    unsigned char hands;
    unsigned char d;
    struct graph *gary[20];
};
typedef struct graph TGraph;
//============================================================================
unsigned char usedflags[100000];
short Stamp;
//============================================================================
void prnnode(TGraph *gptr)
{
    short j;

    printf("node=%d, hands=%d, ", gptr->id, gptr->hands);
    for (j=0; j<gptr->hands; j++)
        printf("%d ", gptr->gary[j]->id);
    printf("\n");
}
//----------------------------------------------------------------------------
void setflag(long n)
{
    usedflags[n>>3] |= 1<<(n & 0x7);
}
//----------------------------------------------------------------------------
int getflag(long n)
{
    return (usedflags[n>>3] & 1<<(n & 0x7));
}
//----------------------------------------------------------------------------
short search(TGraph *gptr, TGraph *fptr)
{
    short i;

    if (gptr==fptr)
        return True;
    //setflag(gptr->id);
    usedflags[gptr->id]=True;
#ifdef DESKTOP
   //prnnode(gptr);
#endif
    for (i=0; i<gptr->hands; i++){
        //if (!getflag(gptr->gary[i]->id)){
        if (!usedflags[gptr->gary[i]->id]){
            if (search(gptr->gary[i], fptr))
                return True;
        }
    }
    /*for (i=0; i<fptr->hands; i++){
        if (search(fptr->gary[i], gptr))
            return True;
    }*/
    return False;
}
//----------------------------------------------------------------------------
void addfrend(TGraph *gptr, TGraph *fptr)
{
    //TGraph **ptr; short i;

    //ptr=malloc(succ(gptr->hands)*sizeof(TGraph**));
    //for (i=0; i<gptr->hands; i++)
    //    ptr[i]=gptr->gary[i];

    //if (gptr->gary)
    //    free(gptr->gary);
    //gptr->gary=ptr;
    gptr->gary[gptr->hands++]=fptr;
}
//----------------------------------------------------------------------------
void filld(TGraph *gptr, long n)
{
    long i;

    for (i=0; i<n; i++)
        gptr[i].d=0;
}
//----------------------------------------------------------------------------
void input(FILE *file)
{
    long i,j,n,m,q,s,t; char buffer[20]; TGraph *graphary,*gptr,*fptr;

    fgets(buffer, sizeof(buffer)-1, file);
    sscanf(buffer, "%ld %ld", &n, &m);
    graphary=malloc(n*sizeof(TGraph));
    memset(graphary, 0, sizeof(TGraph));
    for (i=Stamp=0; i<m; i++){
        fscanf(file, "%ld", &s);
        fscanf(file, "%ld", &t);
        gptr=&graphary[s];
        fptr=&graphary[t];
        gptr->id=s;
        fptr->id=t;
        addfrend(gptr, fptr);
        addfrend(fptr, gptr);
    }
#ifdef DESKTOP
    for (i=m=0; i<n; i++){
        //prnnode(&graphary[i]);
        if (m<graphary[i].hands)
            m=graphary[i].hands;
    }
    setflag(0);
    printf("max hands = %d, %d\n", m, getflag(0));
#endif
    fscanf(file, "%ld", &q);
    for (i=0; i<q; i++){
        fscanf(file, "%ld", &s);
        fscanf(file, "%ld", &t);
        //filld(graphary, n);
        //memset(usedflags, 0, succ(n>>3));
        memset(usedflags, 0, n);
        printf("%s\n", (search(&graphary[s], &graphary[t]))?"yes":"no");
    }
}
//============================================================================
#ifndef DESKTOP
int main()
{
    input(stdin);
    return 0;
}
#endif

C++: #include <iostream>
#include <vector>
#include <unordered_set>
#include <algorithm>
//#include "../cxx-prettyprint/prettyprint.hpp"

std::vector<std::vector<int>> Adj;
std::unordered_set<int> nodes_visited;

bool is_reachable(int s, int t) {
    if (std::find(Adj[s].begin(), Adj[s].end(), t) != Adj[s].end()) {
        return true;
    } else if (nodes_visited.find(s) != nodes_visited.end()) {
        return false;
    } else {
        nodes_visited.insert(s);
        bool value = std::any_of(Adj[s].begin(), Adj[s].end(),
                                 [&](int next){return is_reachable(next, t);});
        return value;
    }
}

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

    Adj.assign(n, std::vector<int>());
    for (int i = 0; i < m; i++) {
        int s, t;
        std::cin >> s >> t;
        Adj[s].push_back(t);
        Adj[t].push_back(s);
    }

    int q;
    std::cin >> q;
    for (int i = 0; i < q; i ++) {
        int s, t;
        std::cin >> s >> t;
        std::cout << (is_reachable(s, t) ? "yes" : "no") << std::endl;
    }
}

C++: #include <cstdint>
#include <vector>

using namespace std;

#define GRAPH_SIZE 100000

enum state_t {
  WHITE,
  GRAY,
  BLACK,
};

struct vertex_t {
  uint32_t color;
  vector<uint32_t> connections;
};

static vertex_t graph[GRAPH_SIZE];
static state_t state[GRAPH_SIZE];

static void colorize(uint32_t index, uint32_t color)
{
  if (state[index] == WHITE) {
    state[index] = GRAY;
    graph[index].color = color;
    for (uint32_t i = 0; i < graph[index].connections.size(); i++) {
      colorize(graph[index].connections[i], color);
    }
    state[index] = BLACK;
  }
}

void check_connection(size_t num)
{
  for (uint32_t i = 0; i < num; i++) {
    state[i] = WHITE;
  }

  uint32_t color = 1;
  for (uint32_t i = 0; i < num; i++) {
    colorize(i, color);
    color++;
  }
}

#include <iostream>

int32_t main()
{
  uint32_t n, m;
  cin >> n >> m;

  for (uint32_t i = 0; i < m; i++) {
    uint32_t index, to;
    cin >> index >> to;
    graph[index].connections.push_back(to);
    graph[to].connections.push_back(index);
  }

  check_connection(n);

  uint32_t qs;
  cin >> qs;

  for (uint32_t i = 0; i < qs; i++) {
    uint32_t from, to;
    cin >> from >> to;

    if (graph[from].color == graph[to].color) {
      cout << "yes" << endl;
    } else {
      cout << "no" << endl;
    }
  }

  return 0;
}