Building Teams

import sys

n, m = map(int, input().split())

friendships = [[] for _ in range(n)]
groups = [0 for _ in range(n)]
visited = [False for i in range(n)]
aprint = True

# Link all friends together
for i in range(m):
    a, b = [int(x) for x in input().split()]
    friendships[a-1].append(b-1)
    friendships[b-1].append(a-1)

# Depth first search through all friends and assign a team to the next friend
def dfs(s):
    stack = [s]
    while stack:
        s = stack.pop()
        if visited[s]: continue
        visited[s] = True
        for u in friendships[s]:
            stack.append(u)
            if visited[u] and groups[u] == groups[s]:
                print("IMPOSSIBLE")
                return False
            if groups[u] == 0: groups[u] = 1 if groups[s] == 2 else 2 
    return True

for i in range(n):
    if groups[i] == 0: groups[i] = 1
    if not dfs(i): 
        aprint = False
        break
if aprint: print(" ".join(map(str, groups)))

Test 1

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 3, in <module>
    n, m =...

Test 2

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 3, in <module>
    n, m =...

Test 3

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 3, in <module>
    n, m =...

Test 4

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 3, in <module>
    n, m =...

Test 5

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 3, in <module>
    n, m =...

Test 6

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 3, in <module>
    n, m =...

Test 7

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 3, in <module>
    n, m =...

Test 8

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 3, in <module>
    n, m =...

Test 9

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 3, in <module>
    n, m =...

Test 10

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 3, in <module>
    n, m =...

Test 11

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 3, in <module>
    n, m =...

Test 12

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 3, in <module>
    n, m =...