Strings
Continue with the data structure you use every day but rarely look inside.
- 19 Lessons
- 19 Problems
- 5 Patterns
- 76 Illustrations
- 38 Slides
Dry-run engine
Reverse a String In Place
Every character is just a byte sitting in a contiguous buffer.
- Input · String
- s = "stressed"
- Output · Returns
- Press play to run it
How it works
While L < R, swap buf[L] and buf[R] in place, then step both pointers inward (L++, R--). Each cell shows its character and the byte it is stored as.
char_buffer[8] · ASCII
- Left (L)0 → 's' (0x73)
- Right (R)7 → 'd' (0x64)
- Buffer"stressed"
Strings are immutable in JavaScript, Java and Python, so the bytes are first copied into a mutable char[] — one byte per ASCII character. L=0 holds 's' (0x73), R=7 holds 'd' (0x64). Since L < R, swap them in place, then move L → 1 and R → 6.
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What you will learn
- How characters become bytes, and why length and index can disagree
- What immutability costs you, and when to reach for a builder instead
- The two pointer pattern applied to reversal, palindromes, and partitioning
- Sliding windows over character frequency maps
- Anagram and grouping problems solved by canonical signatures
- How substring search moves from O(N·M) brute force to O(N + M)
Why this course
A string looks like the easiest data structure in the language until a problem asks you to reverse it in place, and you discover str[i] = 'a' silently does nothing. Strings are arrays with extra rules — an encoding underneath, and in most languages immutability on top — and both rules change which solutions are even available to you.
This course starts at the byte level, shows you what your language actually allocates when you concatenate in a loop, then works through the five patterns that cover nearly every string question in an interview. Each pattern is introduced on a small example before you apply it to progressively harder problems.
Requirements
- Comfort writing loops, conditionals, and functions in at least one language
- The Arrays course, or equivalent familiarity with indexing and two pointers
- No prior knowledge of character encodings — they are introduced from scratch
Course Contents
2. Immutability and building
3. Pattern: Two pointers
4. Pattern: Sliding window
5. Pattern: Frequency counting
6. Pattern: Substring search
7. Pattern: Parsing and partitioning
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