9.15 Interview Questions

Conceptual Questions

• Why are Python strings immutable, and what does that mean for methods like .replace()?
• What’s the difference between find() and index()?
• Why is s[::-1] the idiomatic way to reverse a string when there’s no .reverse() method?
• Why is building a string with += in a loop worse than using .join()?
• What’s the difference between str.format() and an f-string?

Scenario-Based Questions

• A script processes a multi-gigabyte log file and needs to build a filtered output string line by line — how should it be built to avoid a performance cliff?
• An API response sometimes has extra whitespace or inconsistent casing in a status field — how would you normalize it reliably before comparing it?
• A regex pattern used inside a hot loop over thousands of log lines is running slowly — what’s the first optimization to check?

Coding Questions

• Check whether a given string is a palindrome, ignoring case and punctuation.
• Check whether two strings are anagrams of each other.
• Find the longest common prefix shared by a list of strings.
• Find the first non-repeating character in a string.
• Implement basic run-length string compression.

def longest_common_prefix(strs):
    if not strs:
        return ""
    prefix = strs[0]
    for s in strs[1:]:
        while not s.startswith(prefix):
            prefix = prefix[:-1]
    return prefix

>>> longest_common_prefix(["flower", "flow", "flight"])
'fl'

Quick Interview Answer

“String interview questions cluster around three things: confirming immutability is understood (every method returns a new object; .replace() on its own does nothing observable), whether the standard idioms are known instead of hand-rolled loops (s[::-1] for reversal, sorted(a) == sorted(b) for anagrams, ''.join(...) over += for building large strings), and classic coding problems — palindrome, anagram, longest common prefix, first non-repeating character — that test comfort combining slicing, Counter, and sorted() rather than writing everything from scratch.”

Common Mistakes

• Answering “strings are immutable so they can’t be modified at all” without the follow-up: every method still returns a new string, so s.strip() is very much useful — just not in place.
• Solving the longest-common-prefix question with nested loops comparing every character position across every string, instead of the simpler shrink-a-candidate-prefix approach.
• Reaching for a manual character-count loop for the anagram check instead of the one-line sorted(a) == sorted(b) idiom.