Course Schedule

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.util.Arrays;
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[] head = new int[n];
        Arrays.fill(head, -1);

        int[] to = new int[m];
        int[] next = new int[m];
        int[] indegree = new int[n];

        for (int i = 0; i < m; i++) {
            int a = input.nextInt() - 1;
            int b = input.nextInt() - 1;

            to[i] = b;
            next[i] = head[a];
            head[a] = i;

            indegree[b]++;
        }

        int[] queue = new int[n];
        int front = 0;
        int back = 0;

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

        int[] order = new int[n];
        int count = 0;

        while (front < back) {
            int course = queue[front++];

            order[count++] = course;

            for (int edge = head[course];
                 edge != -1;
                 edge = next[edge]) {

                int nextCourse = to[edge];

                indegree[nextCourse]--;

                if (indegree[nextCourse] == 0) {
                    queue[back++] = nextCourse;
                }
            }
        }

        if (count != n) {
            System.out.println("IMPOSSIBLE");
            return;
        }

        StringBuilder sb = new StringBuilder();

        for (int i = 0; i < n; i++) {
            sb.append(order[i] + 1).append(' ');
        }

        System.out.println(sb);
    }

    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: 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