Strings & Hashing

Concept

A hash map stores key → value pairs. A hash function turns the key into a bucket index, giving average O(1) insert, delete and lookup.

Use counting maps for anagrams, frequency problems and grouping.

Strings are immutable in Python and Java: building a string in a loop with + is O(n²); use a list and join, or StringBuilder.

Complexity

Code (Python)

from collections import Counter, defaultdict

def is_anagram(a, b):
    return Counter(a) == Counter(b)

def group_anagrams(words):
    groups = defaultdict(list)
    for w in words:
        groups["".join(sorted(w))].append(w)
    return list(groups.values())

Remember

• “Have I seen this before?” → set.
• “How many times?” → counter map.
• Sorted string or char-count tuple makes a good grouping key.