Brexit

import java.util.ArrayList;
import java.util.Arrays;

/*
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 */
/**
 *
 * @author ahuotala
 */
public class Brexit {
    static ArrayList<Integer>[] list;
    static boolean[] left;
    static int[] limits;
    static int[] neighbours;

    // Eispä
    public static void main(String[] args) {
        IO io = new IO();
        int countries = io.nextInt();
        int connections = io.nextInt();
        int home = io.nextInt();
        int firstToLeave = io.nextInt();
        list = new ArrayList[countries + 1];
        for (int i = 0; i < connections; i++) {
            int n = io.nextInt();
            int m = io.nextInt();
            if (list[n] == null) {
                list[n] = new ArrayList();
            }
            if (list[m] == null) {
                list[m] = new ArrayList();
            }
            list[n].add(m);
            list[m].add(n);
        }

        left = new boolean[countries + 1];
        limits = new int[countries + 1];
        neighbours = new int[countries + 1];
        for (int i = 1; i < limits.length; i++) {
            limits[i] = list[i].size() / 2;
            neighbours[i] = list[i].size();
        }
        dfs(firstToLeave);
        neighbours[firstToLeave]--;
        io.println(left[home] ? "leave" : "stay");
        io.close();
    }
    
    private static void dfs(int n) {
        left[n] = true;
        for (Integer neighbour : list[n]) {
            // does neighbour leave?
            if (!left[neighbour] && neighbourLeftIleave(n, neighbour)) {
                dfs(neighbour);
            }
        }
    }
    
    private static boolean neighbourLeftIleave(int didLeave, int checkLeave) {
        neighbours[checkLeave]--;
        if (neighbours[checkLeave] <= limits[checkLeave]) {
            left[checkLeave] = true;
            return true;
        }
        return false;
    }

}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: ACCEPTED

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED

Test 11

Verdict: ACCEPTED

Test 12

Verdict: ACCEPTED

Test 13

Verdict: ACCEPTED

Test 14

Verdict: ACCEPTED

Test 15

Verdict: ACCEPTED

Test 16

Verdict: ACCEPTED

Test 17

Verdict: ACCEPTED

Test 18

Verdict: ACCEPTED

Test 19

Verdict: ACCEPTED

Test 20

Verdict: ACCEPTED

Test 21

Verdict: ACCEPTED

Test 22

Verdict: ACCEPTED

Test 23

Verdict: ACCEPTED

Test 24

Verdict: ACCEPTED

Test 25

Verdict: ACCEPTED

Test 26

Verdict: ACCEPTED

Test 27

Verdict: ACCEPTED

Test 28

Verdict: ACCEPTED

Test 29

Verdict: ACCEPTED

Test 30

Verdict: ACCEPTED

Test 31

Verdict: ACCEPTED

Test 32

Verdict: ACCEPTED

Test 33

Verdict: ACCEPTED

Test 34

Verdict: ACCEPTED

Test 35

Verdict: ACCEPTED