How would you check if a string is a palindrome, detect anagrams, or find the longest substring without repeating characters?
String manipulation questions appear in nearly every coding interview. They test your ability to work with character-level operations, use hash maps efficiently, and handle edge cases like empty strings, unicode, and case sensitivity.
For palindromes: use two pointers from both ends moving inward, comparing characters. O(n) time, O(1) space.
For anagrams: sort both strings and compare, or use a frequency counter hash map for O(n) time.
For longest substring without repeats: use the sliding window technique with a hash set tracking characters in the current window.
Always discuss edge cases: empty strings, single characters, unicode, case sensitivity.
For palindrome checking, I use two pointers starting at the first and last characters, moving inward. If all character pairs match, it is a palindrome — O(n) time, O(1) space. For anagram detection, I build a frequency map of the first string, then decrement counts with the second string; if all counts are zero, they are anagrams — O(n) time, O(k) space where k is the alphabet size. For the longest substring without repeating characters, I use a sliding window with a hash set. I expand the right pointer, adding characters to the set. When a duplicate is found, I shrink the window from the left until the duplicate is removed. I track the maximum window size throughout. This runs in O(n) time and O(min(n, m)) space where m is the character set size. I always clarify with the interviewer whether we need to handle unicode, whitespace, or case-insensitive comparisons.
Sliding window is the go-to pattern for substring problems — master it.
Always ask about case sensitivity and character set (ASCII vs unicode) before coding.
String immutability varies by language — in Java/Python, building a new string in a loop is O(n²); use StringBuilder or list join.
Sliding window maintains a dynamic range in an array or string using two pointers, expanding and shrinking to satisfy constraints efficiently.
Use language-specific unicode-aware methods. In Python, strings are unicode by default. In Java, use codePoints() instead of charAt() for supplementary characters.
Conceptually similar, but palindrome checking with two pointers is more efficient since you can exit early on mismatch without creating a reversed copy.
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