Connect cities

from collections import defaultdict


def dfs(node: int, visited: list, adj):
    if visited[node - 1]:
        return
    visited[node - 1] = True
    for neighbor in adj[node]:
        if not visited[neighbor - 1]:
            dfs(neighbor, visited, adj)


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

adj = defaultdict(list)
result = []

for i in range(m):
    a, b = [int(x) for x in input().split()]
    adj[a].append(b)
    adj[b].append(a)

visited = [False] * n

dfs(1, visited, adj)

while True:
    try:
        node = visited.index(False) + 1
    except ValueError:
        break
    result.append((1, node))
    dfs(node, visited, adj)

print(len(result))
for road in result:
    print(f"{road[0]} {road[1]}")

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "/box/input/code.py", line 33, in <module>
    dfs(node, visited, adj)
  File "/box/input/code.py", line 10, in dfs
    dfs(neighbor, visited, adj)
  File "/box/input/code.py", line 10, in dfs
    dfs(neighbor, visited, adj)
  File "/box/input/code.py", line 10, in dfs
    dfs(neighbor, visited, adj)
  [Previous line repeated 996 more times]
RecursionError: maximum recursion depth exceeded

Test 7

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "/box/input/code.py", line 33, in <module>
    dfs(node, visited, adj)
  File "/box/input/code.py", line 10, in dfs
    dfs(neighbor, visited, adj)
  File "/box/input/code.py", line 10, in dfs
    dfs(neighbor, visited, adj)
  File "/box/input/code.py", line 10, in dfs
    dfs(neighbor, visited, adj)
  [Previous line repeated 996 more times]
RecursionError: maximum recursion depth exceeded

Test 8

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "/box/input/code.py", line 25, in <module>
    dfs(1, visited, adj)
  File "/box/input/code.py", line 10, in dfs
    dfs(neighbor, visited, adj)
  File "/box/input/code.py", line 10, in dfs
    dfs(neighbor, visited, adj)
  File "/box/input/code.py", line 10, in dfs
    dfs(neighbor, visited, adj)
  [Previous line repeated 996 more times]
RecursionError: maximum recursion depth exceeded

Test 9

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "/box/input/code.py", line 25, in <module>
    dfs(1, visited, adj)
  File "/box/input/code.py", line 10, in dfs
    dfs(neighbor, visited, adj)
  File "/box/input/code.py", line 10, in dfs
    dfs(neighbor, visited, adj)
  File "/box/input/code.py", line 10, in dfs
    dfs(neighbor, visited, adj)
  [Previous line repeated 995 more times]
  File "/box/input/code.py", line 8, in dfs
    for neighbor in adj[node]:
RecursionError: maximum recursion depth exceeded in comparison

Test 10

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "/box/input/code.py", line 25, in <module>
    dfs(1, visited, adj)
  File "/box/input/code.py", line 10, in dfs
    dfs(neighbor, visited, adj)
  File "/box/input/code.py", line 10, in dfs
    dfs(neighbor, visited, adj)
  File "/box/input/code.py", line 10, in dfs
    dfs(neighbor, visited, adj)
  [Previous line repeated 995 more times]
  File "/box/input/code.py", line 8, in dfs
    for neighbor in adj[node]:
RecursionError: maximum recursion depth exceeded in comparison

Test 11

Verdict: TIME LIMIT EXCEEDED

Test 12

Verdict: ACCEPTED