Course Schedule

import sys

def solve():
    input_data = sys.stdin.read().split()
        
    n = int(input_data[0])
    m = int(input_data[1])
    
    graph = [[] for _ in range(n + 1)]
    
    it = iter(input_data[2:])
    for u, v in zip(it, it):
        graph[int(u)].append(int(v))
        

    state = [0] * (n + 1)
    order = []
    
    for i in range(1, n + 1):
        if state[i] == 0:
            stack = [i]
            
            while stack:
                node = stack[-1]
                
                if state[node] == 0:
                    state[node] = 1
                    for neighbor in graph[node]:
                        if state[neighbor] == 1:
                            print("IMPOSSIBLE")
                            return
                        elif state[neighbor] == 0:
                            stack.append(neighbor)
                            
                elif state[node] == 1:
                    state[node] = 2
                    order.append(node)
                    stack.pop()
                    
                else:
                    stack.pop()
                
    order.reverse()
    print(*(order))

if __name__ == '__main__':
    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: ACCEPTED

Test 15

Verdict: ACCEPTED

Test 16

Verdict: ACCEPTED

Test 17

Verdict: ACCEPTED