Chapter 01 - Topic 1.5

Strings and Character Arrays

Strings often behave like arrays of characters, but languages treat them differently. Understanding memory layout and immutability explains why some string operations are cheap while others are expensive.

strings and char arraysstring immutability tutorialanagram frequency mappalindrome string two pointerchar array memory layout

Definition

A string is an ordered sequence of characters, often stored internally like a character array but frequently exposed with language-specific immutability rules.

Core Idea

C-style strings are character arrays ending with a null terminator.

Java and Python strings are immutable, so repeated concatenation creates new strings and can become costly.

Many interview problems on palindromes, anagrams, and substring matching rely on two pointers or frequency maps.

Detailed Explanation

String questions often look different from array questions, but underneath they frequently use the same logic. A string is still an ordered sequence, so indexing, scanning, two pointers, and windows still apply.

What changes is mutability. In many languages, changing a string means creating a new string. That affects performance and explains why builders, arrays of characters, or lists are often used for efficient construction.

This topic is important because many interview problems are really hidden counting and indexing problems: count characters, compare mirrored positions, build maps, or scan for repeating patterns.

Anagram check with a frequency array

JavaScript

function isAnagram(s, t) {
  if (s.length !== t.length) return false;

  const freq = new Array(26).fill(0);

  for (let i = 0; i < s.length; i++) {
    freq[s.charCodeAt(i) - 97]++;
    freq[t.charCodeAt(i) - 97]--;
  }

  return freq.every((count) => count === 0);
}

console.log(isAnagram("listen", "silent"));
console.log(isAnagram("hello", "world"));

Explanation

The array tracks how many times each lowercase letter appears. Characters from the first string increase the count, and characters from the second string decrease it. If both strings contain the same letters with the same frequency, every count finishes at zero.

Output

true
false

Real-life Example

A puzzle or word-game app can check whether two guessed words contain exactly the same letters in a different order.

When To Use It

Use frequency maps when the question depends on how often characters appear.
Use two pointers for palindrome and reverse operations.
Use builders or mutable lists when repeatedly constructing text.

Common Mistakes

Concatenating strings repeatedly in a loop without thinking about cost.
Assuming strings are mutable in every language.
Forgetting to normalize case or strip non-alphanumeric characters when the problem demands it.

Key Takeaways

Strings may look simple, but memory behavior and immutability strongly affect performance.
Use frequency arrays or hash maps when the problem asks for character counts.
Avoid repeated concatenation in loops; use StringBuilder or join-based approaches when available.

Practice Ideas

Check if two words are anagrams.
Reverse a string in-place using a char array.
Find the first non-repeating character.

Frequently Asked Questions

Why are strings often expensive to modify?

Because many languages treat strings as immutable, so modification usually creates a brand-new string object.

When should I use a frequency map?

Use it when the question depends on how many times each character appears, such as anagram or first unique character problems.

Strings and Character Arrays

Definition

Core Idea

Detailed Explanation

Anagram check with a frequency array

When To Use It

Common Mistakes

Key Takeaways

Practice Ideas

Frequently Asked Questions

Why are strings often expensive to modify?

When should I use a frequency map?

More DSA Topic Pages

Strings and Character Arrays

Definition

Core Idea

Detailed Explanation

Anagram check with a frequency array

When To Use It

Common Mistakes

Key Takeaways

Practice Ideas

Frequently Asked Questions

Why are strings often expensive to modify?

When should I use a frequency map?

More DSA Topic Pages