The "Strings Differ by One Character" interview question tasks you with finding if there are at least two strings in a given list that are identical except for exactly one character at the same position. For example, "abcd" and "abzd" differ only at the third character. Given a list of many strings, all of the same length, you need to efficiently determine if such a pair exists. This is a variation of the "near-duplicate detection" problem often found in data processing.
This problem is asked by companies like Okta and Airbnb because it tests a candidate's ability to optimize a search process. A brute-force approach (comparing every pair of strings) takes O(N^2 * L) time, which is too slow for large inputs. The challenge is to find a way to compare the strings in a more efficient manner, usually involving hashing or specialized data structures. It evaluates the candidate's understanding of time-space trade-offs and their ability to use hash functions to represent complex data.
This problem can be solved using Hash Tables with Rolling Hashes or a "Placeholder" technique. In the placeholder approach, for each string, you generate all possible versions with one character replaced by a wildcard (e.g., "abcd" becomes "bcd", "acd", "abd", "abc"). You store these in a Hash Set. If you ever encounter a wildcard string that is already in the set, you've found two strings that differ by exactly one character. This shifts the complexity to O(N * L), where N is the number of strings and L is their length.
Input: ["cat", "bat", "car"]
A frequent mistake is using a brute-force O(N^2) comparison, which fails on large test cases. Another mistake is not correctly calculating the hash when using rolling hashes, leading to collisions or incorrect results. Some candidates also forget that the strings must be of the same length and that the difference must be at the same index. Using too much memory by storing all wildcard variations without consideration for the constraints is also a potential pitfall.
To excel in the Strings Differ by One Character interview question, familiarize yourself with different hashing techniques. Practice the "wildcard" or "placeholder" pattern, as it is a powerful tool for solving "off-by-one" problems. Also, be prepared to discuss the memory implications of your solution, as storing many variations of strings can significantly increase the space complexity.
| Title | Difficulty | Topics | LeetCode |
|---|---|---|---|
| Check If a String Contains All Binary Codes of Size K | Medium | Solve | |
| Unique Substrings With Equal Digit Frequency | Medium | Solve | |
| Repeated DNA Sequences | Medium | Solve | |
| Encode and Decode TinyURL | Medium | Solve | |
| Minimum Time to Revert Word to Initial State I | Medium | Solve |
