The base version was fine, two-pointer from both ends after cleaning the string.
Start by clarifying the requirements: define alphanumeric (e.g., letters and digits) and case normalization (e.g., lowercase). Then present a two-pointer approach that filters and compares characters in-place, discussing time and space complexity. Finally, mention alternative methods like using regular expressions or building a filtered string, and trade-offs between them.
Pro tip: Demonstrate awareness of Unicode and locale-specific characters, and mention that in production you'd use a well-tested library function rather than reimplementing, but here you're showing algorithmic thinking.
Ask about the definition of alphanumeric (ASCII vs Unicode), case normalization (lowercase vs casefold), and handling of empty strings or strings with no alphanumeric characters.
Explain that you'll use two pointers starting at the ends, skip non-alphanumeric characters, compare characters after lowercasing, and move inward until they meet.
State that the two-pointer method runs in O(n) time and O(1) extra space, while approaches that build a filtered string use O(n) space. Discuss when each might be preferable.
Write clean code (or pseudocode) and walk through test cases like 'A man, a plan, a canal: Panama' and 'race a car', including edge cases.
Mention how you'd handle Unicode, performance for very large strings, and whether to use built-in functions for production code.
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Clarify the constraints: single pass, sublinear extra space, and streaming input. Then propose a two-pointer approach using a deque or a rolling hash to compare characters from both ends, while discussing trade-offs and edge cases.
Pro tip: Mention that for a true single-pass with sublinear space, you might need to assume random access or use a probabilistic method like hashing, and always discuss the trade-offs between time, space, and accuracy.
Ask about input size, memory limits, whether random access is allowed, and if approximate answers are acceptable.
Use a deque to store characters from the beginning and end, comparing them as they arrive, but note that this may use O(n) space in the worst case.
Suggest using a rolling hash to compute forward and backward hashes incrementally, which uses O(1) space but is probabilistic.
Compare deterministic vs probabilistic methods, handle odd/even length, and mention that exact single-pass with sublinear space may be impossible without assumptions.
Recommend a solution based on the constraints, such as using a rolling hash if approximate is acceptable, or buffering if memory allows.
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k=1 I'd seen before so that went okay, recursive check on mismatched positions.
Start by clarifying the problem: given a string and integer k, determine if it can become a palindrome by deleting at most k characters. Then present a recursive two-pointer approach that generalizes the standard palindrome check, and analyze its time and space complexity, mentioning memoization for efficiency.
Pro tip: Discuss how this problem relates to the Longest Palindromic Subsequence (LPS) and that the minimum deletions needed is n - LPS length, showing you can reframe the problem to leverage known algorithms.
Confirm that deletions can be from any position, k is non-negative, and the goal is to return true if the string can become a palindrome with at most k deletions. Ask about input size and expected time complexity.
Use two pointers from both ends. When characters mismatch, try deleting either the left or right character and check if the remaining substring is a palindrome (using a helper function). This handles k=1 in O(n) time.
Define a recursive function that takes the string and k. At each mismatch, recursively try deleting from left or right with k-1. Base cases: if k<0 return false; if pointers cross return true.
Use memoization on (left, right, k) to avoid recomputation, or reframe as finding the longest palindromic subsequence and checking if n - LPS <= k. Discuss time and space complexity.
Compare recursive vs iterative DP, discuss space optimization, and handle edge cases like empty string, k >= n, and strings with all same characters.
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Returning the mismatch index was straightforward but the unit test discussion caught me a little flat-footed.
Start by clarifying the problem: define palindrome, mismatch, and index pair. Then propose a two-pointer approach that scans from both ends, returning the first mismatch indices if found, or a success indicator if it's a palindrome. Finally, analyze time and space complexity, discuss edge cases, and outline a unit testing strategy.
Pro tip: Mention that in ML pipelines, palindrome checks can be used for data validation or symmetry detection, and emphasize that early termination on mismatch makes the algorithm efficient for large strings.
Confirm what constitutes a palindrome (e.g., case sensitivity, ignoring non-alphanumeric characters) and what to return if the string is a palindrome. Also clarify the expected output format for the index pair.
Use two pointers starting at the beginning and end, moving inward while characters match. Return the indices of the first mismatch, or a sentinel value (e.g., (-1, -1)) if no mismatch is found.
Time complexity is O(n) in the worst case (when the string is a palindrome or mismatch is at the center), but early termination can make it faster. Space complexity is O(1) as only two pointers are used.
Consider empty string, single character, even/odd length, all same characters, mismatch at first/last pair, and strings with special characters or spaces if relevant.
Write tests covering normal cases, edge cases, and performance for large inputs. Use assertions to verify correct index pairs and handle palindrome cases appropriately.
AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.