Group Anagrams — Interview Guide

• Sorting each string gives a canonical key — O(k log k) per string.
• A 26-character count tuple is a key in O(k) per string and is faster for long strings.
• Both produce the same groups; the count key wins when strings are long.

import collections
def group_anagrams(strs):
    groups = collections.defaultdict(list)
    for s in strs:
        key = [0] * 26
        for c in s:
            key[ord(c) - 97] += 1
        groups[tuple(key)].append(s)
    return list(groups.values())

• Handle Unicode strings beyond a-z.
• Return groups sorted by size, then alphabetically.
• Stream version: group anagrams arriving one at a time with bounded memory.