Q3

package hackathon;
//Phanindhar Bodla
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class Eulearian {
    static int first[];
    static int last[];
    static boolean a[][];
    static boolean flag[];

    public static void main(String[] args) throws NumberFormatException,
            IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        int cases = Integer.parseInt(br.readLine());
        int num;
        String input[];
        int firstAt;
        int lastAt;
        while (cases-- > 0) {
            first = new int[100];
            last = new int[100];
            flag = new boolean[100];
            a = new boolean[100][100];
            num = Integer.parseInt(br.readLine());
            for (int i = 0; i < num; i++) {
                input = br.readLine().split(" ");
                firstAt = Integer.parseInt(input[0]);
                lastAt = Integer.parseInt(input[1]);

                first[firstAt]++;

                last[lastAt]++;
                flag[firstAt] = true;
                flag[lastAt] = true;
                a[firstAt][lastAt] = true;
            }
            solve();
        }
    }

    private static void solve() {
        int start = check();
        int wc = 0;
        int cnt = 0;
        int i = 0;
        boolean isPossible = false;
        boolean visited[] = new boolean[100];
        if (start != -1) {
            while (i < 100) {
                if (flag[i])
                    wc++;// total count of the nodes
                i++;
            }
            i = 0;
            while (!flag[i] && i < 100) // pointing i to the first node of our graph
                i++;
            visited[i] = true; // marking it as visited
            connect(i, visited);// this is a method which passes from all the possible edges and mark them visited using Depth first search
            i = 0;
            while (i < 100) { // now we count the number of visited nodes in count cnt
                if (visited[i])
                    cnt++;
                i++;
            }
            if (wc == cnt)
                System.out.println("yes"); // visited count  matches total count
            else
                System.out.println("no"); // some unvisited nodes exist

            isPossible = true;

        }
        if (!isPossible)
            System.out.println("no");
    }

    private static void connect(int ii, boolean[] visited) {
        for (int i = 0; i < 100; i++) {
            if (a[i][ii] || a[ii][i]) {
                if (!visited[i]) {
                    visited[i] = true;
                    connect(i, visited);
                }
            }
        }
    }

    private static int check() {
        int start = -1;
        int end = -1;
        int diff;
        for (int i = 0; i < 100; i++) {
            diff = first[i] - last[i];// difference between no of occurrences at start and end point
            if (diff == 1) { // shows the sign of a the starting point occurrence
                if (start == 1) // which means we already got a start point
                    return -1; // multiple start point not possible in a trail
                start = 1;// here i is the stating point
                continue;
            }
            if (diff == -1) { // shows the sign of a the ending point occurrence
                if (end == 1) //   which means we already got a end point
                    return -1; //  multiple end point not possible in a trail
                end = 1;// here i is the ending point
                continue;
            }
            if (diff > 0) { // diff can't be more than zero for non start and end points
                return -1; // not a trail
            }
        }
        return 0; // could be a trail
    }
}