Course Schedule

import sys 

def solve():
    input = sys.stdin.read().split()

    n = int(input[0])
    m = int(input[1])

    g = [[] for _ in range(n+1)]

    i = 2 
    for _ in range(m):
        a = int(input[i])
        b = int(input[i+1])
        g[a].append(b)
        i += 2 

    state = [0] * (n+1)
    order = []
    possible = True

    def dfs(node): 
        nonlocal possible
        state[node] = 1
        for n in g[node] :
            if state[n] == 1 : 
                possible = False
                return
            elif state[n] == 0 : 
                dfs(n)
                if not possible : 
                    return

        state[node] = 2
        order.append(node)

    for i in range(1,n+1): 
        if state[i] == 0 :
            dfs(i)
            if not possible : 
                break

    if not possible : 
        print("IMPOSSIBLE")
    else: 
        order.reverse()
        print(*order)

if "__main__" == __name__ : 
    solve()

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: RUNTIME ERROR

Traceback (most recent call last):
  File "/box/input/code.py", line 50, in <module>
    solve()
  File "/box/input/code.py", line 39, in solve
    dfs(i)
  File "/box/input/code.py", line 30, in dfs
    dfs(n)
  File "/box/input/code.py", line 30, in dfs
    dfs(n)
  File "/box/input/code.py", line 30, in dfs
    dfs(n)
  [Previous line repeated 1685 more times]
  File "/box/input/code.py", line 26, in dfs
    if state[n] == 1 :
RecursionError: maximum recursion depth exceeded

Test 15

Verdict: ACCEPTED

Test 16

Verdict: ACCEPTED

Test 17

Verdict: ACCEPTED