Connect cities

num_cities, num_roads = [int(x) for x in input().split()]
neightbours = [[] for _ in range(num_cities + 1)]
visited = []

for _ in range(num_roads):
    x1, x2 = [int(x) for x in input().split()]
    neightbours[x1].insert(0, x2)
    neightbours[x2].insert(0, x1)

def dfs(node: int) -> None:
    visited.append(node)
    for x in neightbours[node]:
        if x not in visited:
            dfs(x)

path = []
for x in range(1, num_cities + 1):
    if x not in visited:
        path.append(x)
        dfs(x)

print(f"{len(path) - 1}")
for i in range(1, len(path)):
    print(f"{path[i - 1]} {path[i]}")

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 "input/code.py", line 20, in <module>
    dfs(x)...

Test 7

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 20, in <module>
    dfs(x)...

Test 8

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 20, in <module>
    dfs(x)...

Test 9

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 20, in <module>
    dfs(x)...

Test 10

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "input/code.py", line 20, in <module>
    dfs(x)...

Test 11

Verdict: TIME LIMIT EXCEEDED

Test 12

Verdict: ACCEPTED