Graphs

Model as an adjacency defaultdict(list) or a grid. BFS for shortest hops, DFS for reachability.

Core syntax

• BFS — a deque, plus a visited set.
• Grid neighbors — iterate [(0,1),(0,-1),(1,0),(-1,0)].

from collections import deque
q = deque([start])
seen = {start}
while q:
    node = q.popleft()
    for nxt in graph[node]:
        if nxt not in seen:
            seen.add(nxt)
            q.append(nxt)

Watch out

• Mark visited when enqueuing, not dequeuing, to avoid duplicates.