Connect cities

def task1(n, roads):
    graph = []
    vis = [False] * n

    for i in range(n):
        graph.append([])

    for a, b in roads:
        graph[b - 1].append(a - 1)  # connect b->a
        graph[a - 1].append(b - 1)  # connect a->b

    subgraphs = []
    for i in range(n):
        if not vis[i]:
            subgraph = []
            dfs(i, graph, vis, subgraph)  # dfs connectivity check
            subgraphs.append(subgraph)

    added_roads = []

    for i in range(len(subgraphs) - 1):  # we connect deepest node in each subgrph
        added_roads.append([subgraphs[i][-1] + 1, subgraphs[i + 1][-1] + 1])

    return added_roads


def dfs(s, adj, vis, subgraph):
    if vis[s]:
        return
    vis[s] = True
    subgraph.append(s)
    for u in adj[s]:
        if not vis[u]:
            dfs(u, adj, vis, subgraph)


if __name__ == "__main__":
    n, m = map(int, input().split())
    roads = []
    for i in range(m):
        a, b = map(int, input().split())
        roads.append([a, b])
    added_roads = task1(n, roads)

    print(len(added_roads))
    for road in added_roads:
        print(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 43, in <module>
    a...

Test 7

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "/box/input/code.py", line 43, in <module>
    a...

Test 8

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "/box/input/code.py", line 43, in <module>
    a...

Test 9

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "/box/input/code.py", line 43, in <module>
    a...

Test 10

Verdict: RUNTIME ERROR

Traceback (most recent call last):
  File "/box/input/code.py", line 43, in <module>
    a...

Test 11

Verdict: ACCEPTED

Test 12

Verdict: ACCEPTED