Course Schedule

import sys

n, m = [int(x) for x in input().split()]

GRAPH = {i:[] for i in range(n+1)}
for _ in range(m):
    a,b = [int(x) for x in input().split()]
    GRAPH[a].append(b)

NOT_VISITED = set(range(2, n + 1))
solution = []

def dfs(node):
    '''Depth-first search'''
    to_visit = [node]
    to_visit_set = {node}
    while len(to_visit) > 0:
        node = to_visit[-1]
        if len(GRAPH[node]):
            next_node = GRAPH[node].pop()
            if next_node in NOT_VISITED:
                NOT_VISITED.remove(next_node)
                to_visit.append(next_node)
                to_visit_set.add(next_node)
            elif next_node in to_visit_set:
                print('IMPOSSIBLE')
                sys.exit()
        else:
            to_visit_set.remove(node)
            to_visit.pop()
            solution.append(node)

dfs(1)
while len(NOT_VISITED) > 0:
    node = NOT_VISITED.pop()
    dfs(node)

solution.reverse()
for elt in solution:
    print(elt, end=' ')

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