Go: package main

import (
	"bufio"
	"fmt"
	"os"
	"strconv"
)

var scanner = bufio.NewScanner(os.Stdin)

func nextString() string {
	scanner.Scan()
	return scanner.Text()
}

func nextInt() int {
	n, err := strconv.Atoi(nextString())
	if err != nil {
		fmt.Printf("strconv.Atoi failed: %v\n", err)
	}
	return n
}

func newGraph(nNodes int) *Graph {
	g := Graph{}
	g.nodes = make([]*GraphNode, nNodes)
	for iNode := 0; iNode < len(g.nodes); iNode++ {
		g.nodes[iNode] = &GraphNode{}
		g.nodes[iNode].key = GraphNodeKey(iNode)
	}
	return &g
}

var infty = 1000000000

type GraphNodeKey int

// GraphNode is a node for undirected graph
type GraphNode struct {
	key       GraphNodeKey
	adjacents []*GraphNode
	visited   bool
	d, f      int
	parent    *GraphNode
	cc        int
}

func (g *Graph) setAdjacent(l, r GraphNodeKey) {
	g.nodes[l].adjacents = append(g.nodes[l].adjacents, g.nodes[r])
}

// Graph is a undirected graph
type Graph struct {
	nodes []*GraphNode
}

func (g *Graph) initialize() {
	for iNode := 0; iNode < len(g.nodes); iNode++ {
		g.nodes[iNode].visited = false
		g.nodes[iNode].d = infty
		g.nodes[iNode].f = infty
		g.nodes[iNode].parent = nil
		g.nodes[iNode].cc = -1 // 22.3-12
	}
}

type QueueEntry struct {
	node *GraphNode
	next *QueueEntry
}

type Queue struct {
	head, tail *QueueEntry
}

func (q *Queue) enqueue(node *GraphNode) {
	entry := QueueEntry{}
	entry.node = node

	if q.tail == nil {
		q.head = &entry
		q.tail = &entry
		return
	}

	q.tail.next = &entry
	q.tail = q.tail.next
}

func (q *Queue) dequeue() *QueueEntry {
	if q.head == nil {
		return nil
	}
	poped := q.head
	q.head = q.head.next
	if q.head == nil {
		q.tail = nil
	}
	return poped
}

func (q *Queue) isEmpty() bool {
	return q.head == nil
}

var time = 0

func fillDistancesByDepthFirstSearch(g *Graph) {
	g.initialize()
	time = 0
	cc := 0

	for _, u := range g.nodes {
		if !u.visited {
			dfsVisit(g, u, cc)
			cc++
		}
	}
}

func dfsVisit(g *Graph, u *GraphNode, cc int) {
	time++
	u.d = time
	u.cc = cc

	u.visited = true
	for _, v := range u.adjacents {
		if !v.visited {
			v.parent = u
			dfsVisit(g, v, cc)
		}
	}
	time++
	u.f = time
}

func main() {
	scanner.Split(bufio.ScanWords)

	nUsers := nextInt()
	nEdges := nextInt()
	graph := newGraph(nUsers)
	for iEdge := 0; iEdge < nEdges; iEdge++ {
		l := GraphNodeKey(nextInt())
		r := GraphNodeKey(nextInt())
		graph.setAdjacent(l, r)
		graph.setAdjacent(r, l)
	}

	fillDistancesByDepthFirstSearch(graph)

	nQueries := nextInt()
	for iQuery := 0; iQuery < nQueries; iQuery++ {
		l := GraphNodeKey(nextInt())
		r := GraphNodeKey(nextInt())

		// fmt.Println(graph.nodes[l].d, graph.nodes[r].d, graph.nodes[r].f, graph.nodes[l].f)
		if graph.nodes[l].cc == graph.nodes[r].cc {
			fmt.Println("yes")
		} else {
			fmt.Println("no")
		}
	}
}

C++: using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace _20180307_AOJ
{
    class ALDS1_11D
    {
        static Node[] nodes;
        static void Main(string[] args)
        {
            for (;;)
            {
                string line = Console.ReadLine();
                if (string.IsNullOrEmpty(line))
                {
                    break;
                }

                int[] n_m = line.Split(' ').Select(int.Parse).ToArray();
                int n = n_m[0];
                int m = n_m[1];

                nodes = new Node[n];
                for(int i=0;i<n;i++)
                {
                    nodes[i] = new Node();
                }
                for(int i=0;i<m;i++)
                {
                    int[] s_t = Console.ReadLine().Split(' ').Select(int.Parse).ToArray();
                    int s = s_t[0];
                    int t = s_t[1];
                    nodes[s].Friends.Add(t);
                    nodes[t].Friends.Add(s);
                }

                int q = int.Parse(Console.ReadLine());
                for(int i=0;i<q;i++)
                {
                    int[] s_t = Console.ReadLine().Split(' ').Select(int.Parse).ToArray();
                    int s = s_t[0];
                    int t = s_t[1];
                    bool isFriend = IsFriend(s, t, new List<int> { s });
                    if(isFriend)
                    {
                        Console.WriteLine("yes");
                    }
                    else
                    {
                        Console.WriteLine("no");
                    }
                }
            }
        }

        static bool IsFriend(int s, int t, List<int> visited)
        {
            bool rv = false;
            Node ns = nodes[s];
            if (ns.Friends.Contains(t))
            {
                rv = true;
            }
            else
            {
                foreach (var v in ns.Friends)
                {
                    if (!visited.Contains(v))
                    {
                        visited.Add(v);
                        rv = IsFriend(v, t, visited);
                        if (rv)
                        {
                            break;
                        }
                    }
                }
            }

            return rv;
        }
        
    }

    class Node
    {
        public HashSet<int> Friends { get; set; }
        public Node()
        {
            Friends = new HashSet<int>();
        }
    }
}

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

/* UnionFind：素集合系管理の構造体(union by size)
    isSame(x, y): x と y が同じ集合にいるか。 計算量はならし O(α(n))
    unite(x, y): x と y を同じ集合にする。計算量はならし O(α(n))
    treeSize(x): x を含む集合の要素数。
*/
struct UnionFind {
    vector<int> size, parents;
    UnionFind() {}
    UnionFind(int n) {  // make n trees.
        size.resize(n, 0);
        parents.resize(n, 0);
        for (int i = 0; i < n; i++) {
            makeTree(i);
        }
    }
    void makeTree(int x) {
        parents[x] = x;  // the parent of x is x
        size[x] = 1;
    }
    bool isSame(int x, int y) { return findRoot(x) == findRoot(y); }
    void unite(int x, int y) {
        x = findRoot(x);
        y = findRoot(y);
        if (size[x] > size[y]) {
            parents[y] = x;
            size[x] += size[y];
        } else {
            parents[x] = y;
            size[y] += size[x];
        }
    }
    int findRoot(int x) {
        if (x != parents[x]) {
            parents[x] = findRoot(parents[x]);
        }
        return parents[x];
    }
    int treeSize(int x) { return size[findRoot(x)]; }
};

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

    UnionFind uf(n);
    for (int i = 0; i < m; i++) {
        int a, b;
        cin >> a >> b;
        uf.unite(a, b);
    }

    int q;
    cin >> q;
    vector<int> s(q), t(q);
    for (int i = 0; i < q; i++) {
        cin >> s.at(i) >> t.at(i);
    }

    for (int i = 0; i < q; i++) {
        if (uf.isSame(s[i], t[i])) {
            cout << "yes" << endl;
        } else {
            cout << "no" << endl;
        }
    }

    return 0;
}

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

typedef long long ll;
static const int MAX = 100000;
static const int NIL = -1;

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

// 深さ優先探索 r:頂点, c:色の番号
void dfs(int r, int c) {
	color[r] = c;

	for (int i = 0; i < G[r].size(); i++) {
		int v = G[r][i];
		if (color[v] == NIL) {
			color[v] = c;
			dfs(v, c);
		}
	}
}

void assignColor() {
	int id=1;
	// 色の初期化
	for(int i=0; i<n; i++) color[i]=NIL;
	// 事前にそれぞれの頂点の色分けをする
	for (int u = 0; u < n; u++) {
		// 色分けが行われていない頂点に新しい色を割り当てる
		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;
		}
	}
}