Arrays & Hashing

The bread and butter: lists for ordered data, dicts and sets for O(1) lookup.

Core syntax

• Frequency count — Counter(nums) or a defaultdict(int) with count[x] += 1.
• Group by key — defaultdict(list) then groups[key].append(x).
• Membership — x in seen where seen is a set (O(1), not a list).
• Index map — {v: i for i, v in enumerate(nums)}.

from collections import Counter, defaultdict
freq = Counter(nums)                 # {val: count}
groups = defaultdict(list)
for s in strs:
    groups[tuple(sorted(s))].append(s)

Watch out

• {} is an empty dict, not a set — use set().
• dict.get(k, default) avoids KeyError.