Course Schedule

Note: input/CourseSchedule.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.util.ArrayDeque;
import java.util.ArrayList;
import java.util.StringTokenizer;

public class CourseSchedule {
    public static void main(String[] args) {
        FastScanner input = new FastScanner(System.in);
        int n = input.nextInt();
        int m = input.nextInt();

        int[][] requirements = new int[m][2];

        for(int i = 0; i < m; i++){
            requirements[i][0] = input.nextInt();
            requirements[i][1] = input.nextInt();
        }

        int[] output = new int[n];

        ArrayList<Integer>[] graph = new ArrayList[n];
        for (int i = 0; i < n; i++) {
            graph[i] = new ArrayList<>();
        }

        int[] indegree = new int[n];

        for (int i = 0; i < m; i++) {
            int a = requirements[i][0] - 1;
            int b = requirements[i][1] - 1;

            graph[a].add(b);
            indegree[b]++;
        }

        ArrayDeque<Integer> queue = new ArrayDeque<>();

        for (int i = 0; i < n; i++) {
            if (indegree[i] == 0) {
                queue.add(i);
            }
        }

        int count = 0;

        while (!queue.isEmpty()) {
            int course = queue.poll();

            output[count] = course + 1;
            count++;

            for (int next : graph[course]) {
                indegree[next]--;

                if (indegree[next] == 0) {
                    queue.add(next);
                }
            }
        }

        if (count != n) {
            System.out.println("IMPOSSIBLE");
        } else {
            for (int i = 0; i < n; i++) {
                System.out.print(output[i] + " ");
            }
        }
    }

    static class FastScanner { //static class fastScanner
    private BufferedReader reader = null;
    private StringTokenizer tokenizer = null;

    public FastScanner(InputStream in) {
        reader = new BufferedReader(new InputStreamReader(in));
        tokenizer = null;
    }

    public String next() {
        if (tokenizer == null || !tokenizer.hasMoreTokens()) {
            try {
                tokenizer = new StringTokenizer(reader.readLine());
            } catch (IOException e) {
                throw new RuntimeException(e);
            }
        }
        return tokenizer.nextToken();
    }

    public String nextLine() {
        if (tokenizer == null || !tokenizer.hasMoreTokens()) {
            try {
                return reader.readLine();
            } catch (IOException e) {
                throw new RuntimeException(e);
            }
        }

        return tokenizer.nextToken("\n");
    }

    public long nextLong() {
        return Long.parseLong(next());
    }

    public int nextInt() {
        return Integer.parseInt(next());
    }

    public double nextDouble() {
    	 return Double.parseDouble(next());
	 }

    public int[] nextIntArray(int n) {
        int[] a = new int[n];
        for (int i = 0; i < n; i++)
            a[i] = nextInt();
        return a;
    }

    public long[] nextLongArray(int n) {
        long[] a = new long[n];
        for (int i = 0; i < n; i++)
            a[i] = nextLong();
        return a;
    }
}
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: TIME LIMIT EXCEEDED

Test 7

Verdict: TIME LIMIT EXCEEDED

Test 8

Verdict: TIME LIMIT EXCEEDED

Test 9

Verdict: TIME LIMIT EXCEEDED

Test 10

Verdict: ACCEPTED

Test 11

Verdict: TIME LIMIT EXCEEDED

Test 12

Verdict: ACCEPTED

Test 13

Verdict: ACCEPTED

Test 14

Verdict: TIME LIMIT EXCEEDED

Test 15

Verdict: TIME LIMIT EXCEEDED

Test 16

Verdict: ACCEPTED

Test 17

Verdict: ACCEPTED