Five Brain-Bending Python Interview Challenges: Hard MCQ Quiz — Questions & Answers

This quiz contains 5 questions. Below is a complete reference of all questions, answer choices, and correct answers. You can use this section to review after taking the interactive quiz above.

1. Question 1: Reverse only if all characters are unique

   Which one-line Python expression reverses a string s only when all of its characters are unique, without using slicing [::-1] or reversed(), for example producing 'cba' from s='abc' but leaving s='abca' unchanged?

   • ''.join(s[i] for i in range(len(s)-1, -1, -1)) if len(s) == len(set(s)) else s
   • ''.join(reversed(s)) if len(s) == len(set(s)) else s
   • s[::-1] if len(set(s)) == len(s) else s
   • ''.join(s[i] for i in range(len(s))) if len(set(s)) == len(s) else s
   • s if len(set(s)) == len(s) else ''.join(s[i] for i in range(len(s)-1, -1, -1))
   Show correct answer

   Correct answer: ''.join(s[i] for i in range(len(s)-1, -1, -1)) if len(s) == len(set(s)) else s

2. Question 2: Second-largest in one pass without sorting

   Given a list nums and sentinels largest = second = float('-inf'), which in-loop update logic correctly computes a distinct second-largest value in a single pass without using sort or sorted, even when nums contains duplicates (e.g., nums=[5, 1, 5, 3])?

   • if x > largest: second, largest = largest, x; elif x < largest and x > second: second = x
   • if x >= largest: second, largest = largest, x; elif x > second: second = x
   • if x > largest: largest, second = x, second; elif x > second: second = x
   • if x > largest: second = x; largest = x
   • if x > largest and x > second: second = x; largest = x
   Show correct answer

   Correct answer: if x > largest: second, largest = largest, x; elif x < largest and x > second: second = x

3. Question 3: Flatten a shallow nested list without recursion in one line

   Which one-line Python expression flattens a list like lst = [1, [2, 3], [4, 5], 6] into [1, 2, 3, 4, 5, 6] without using recursion or additional imports?

   • [y for x in lst for y in (x if isinstance(x, list) else [x])]
   • [item for sub in lst for item in sub]
   • list(sum(lst))
   • list(chain.from_iterable(lst))
   • flatten(lst)
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   Correct answer: [y for x in lst for y in (x if isinstance(x, list) else [x])]

4. Question 4: Understanding reverse indices with range

   For a string s of length 4, what sequence of indices does range(len(s)-1, -1, -1) generate, which is commonly used to traverse s in reverse without slicing?

   • [3, 2, 1, 0]
   • [4, 3, 2, 1]
   • [3, 2, 1]
   • [0, 1, 2, 3]
   • [-1, -2, -3, -4]
   Show correct answer

   Correct answer: [3, 2, 1, 0]

5. Question 5: Applying reverse-if-unique to a non-unique string

   When evaluating the expression ''.join(s[i] for i in range(len(s)-1, -1, -1)) if len(s) == len(set(s)) else s on s = 'abca', what string is returned?

   • abca
   • acba
   • aabc
   • a c b a
   Show correct answer

   Correct answer: abca