Template: with — beginners get stuck here before they can attempt BFS / DFS.

with — beginners get stuck here before they can attempt BFS / DFS.

from collections import defaultdict


def build_graph(edges, n_nodes=None, directed=False):
    """Convert an edge list to an adjacency list.

    Args:
        edges: list of [u, v] pairs. For weighted graphs, pass [u, v, w].
        n_nodes: optional number of nodes; if provided, ensures isolated nodes
                 (nodes with no edges) are still present in the adjacency dict.
        directed: True for directed graphs (one-way edges).

    Returns:
        defaultdict(list) where graph[u] = list of neighbours.
    """
    graph = defaultdict(list)
    if n_nodes is not None:
        for i in range(n_nodes):
            graph[i] = []
    for edge in edges:
        if len(edge) == 2:
            u, v = edge
            graph[u].append(v)
            if not directed:
                graph[v].append(u)
        elif len(edge) == 3:
            u, v, w = edge
            graph[u].append((v, w))
            if not directed:
                graph[v].append((u, w))
    return graph