04. Arrays & Strings

Arrays and strings are the foundation for two-pointer, palindrome, and anagram problems. Master slicing, the string methods you'll use every day, and ASCII-based tricks.

arraystringtwo-pointer

Duration

⏱ ~1 hour

Level

📊 Beginner

Prerequisite

🎯 Basics track

OUTCOME

Transform any string/array problem into a clean Python solution.

What you'll learn

1Use slicing, reversing, and stepping confidently
2Apply common string methods: split, join, strip, replace, find
3Solve palindrome and anagram problems

Overview

Most string problems boil down to slicing, indexing, and a small set of methods. ASCII tricks (ord/chr arithmetic) handle Caesar ciphers, character counts, and similar tasks elegantly.

Core Concepts

1. Slicing

python

s = 'abcdefg'
s[0]      # 'a'
s[-1]     # 'g'
s[2:5]    # 'cde'
s[:3]     # 'abc'
s[3:]     # 'defg'
s[::-1]   # 'gfedcba'  reversed
s[::2]    # 'aceg'     every 2nd

2. Essential string methods

Method	Effect	Example
split()	Split on whitespace	'a b c'.split() → ['a','b','c']
split(',')	Split on delimiter	'a,b,c'.split(',') → ['a','b','c']
join(iter)	Concatenate with separator	','.join(['a','b']) → 'a,b'
strip()	Trim whitespace	' hi '.strip() → 'hi'
replace(a, b)	Replace all	'aaa'.replace('a','b') → 'bbb'
find(sub)	Index of first match (or -1)	'hello'.find('ll') → 2
count(sub)	Count occurrences	'banana'.count('a') → 3
isdigit() / isalpha()	Character class checks	'12'.isdigit() → True

3. ASCII arithmetic

python

ord('A')   # 65
ord('a')   # 97
ord('0')   # 48
chr(65)    # 'A'

# Lowercase to uppercase
chr(ord('a') - 32) == 'A'

# 'c' - 'a' = letter index 0..25
ord('c') - ord('a')   # 2

4. Two-pointer pattern (palindrome)

python

def is_palindrome(s):
    i, j = 0, len(s) - 1
    while i < j:
        if s[i] != s[j]:
            return False
        i += 1
        j -= 1
    return True

Examples

Example 1 — src/01_slicing.py

Slicing demos: subranges, reversing, stepping.

Example 2 — src/02_string_methods.py

Daily-use methods: split/join/strip/replace/find.

Example 3 — src/03_palindrome.py

Two-pointer palindrome check, plus the one-liner with s == s[::-1].

Example 4 — src/04_anagram.py

Anagram check with collections.Counter — O(n).

Common Mistakes

Treating strings as mutable. Strings are immutable — use list and ''.join() to build incrementally.
Concatenating in a loop with `+=`. That's O(n²). Use a list and join at the end.
Off-by-one in s[:k] (exclusive) vs s[:k+1].
Forgetting that .strip() returns a new string and doesn't modify in place.

Recap

Slicing covers reversing, stepping, and substring extraction.
Two-pointer is the workhorse pattern for string/array problems.
Counter and ASCII arithmetic turn many problems into 2-line solutions.

Try It

BOJ 10808 (Alphabet count).
BOJ 1316 (Group word checker).

Example code / lecture materials

All lecture materials and example code are openly available on GitHub.

View on GitHub ↗