To solve Longest Uncommon Subsequence II, first sort the strings by length to prioritize longer candidates. Then, scan each string and check if it is a subsequence of any other using hash lookups. Return the length of the first string that is not a subsequence of any other, otherwise return -1 if no uncommon subsequence exists.
Problem Statement
Given an array of strings strs, determine the length of the longest string that is a subsequence of only one string in the array. An uncommon subsequence is a string that appears as a subsequence in one string but not in any other.
A subsequence is formed by deleting zero or more characters from a string without changing the order of the remaining characters. Return -1 if no string satisfies the uncommon subsequence condition.
Examples
Example 1
Input: strs = ["aba","cdc","eae"]
Output: 3
Example details omitted.
Example 2
Input: strs = ["aaa","aaa","aa"]
Output: -1
Example details omitted.
Constraints
- 2 <= strs.length <= 50
- 1 <= strs[i].length <= 10
- strs[i] consists of lowercase English letters.
Solution Approach
Sort Strings by Length Descending
Begin by sorting the array from longest to shortest so that the first valid uncommon subsequence encountered is guaranteed to be the longest. This leverages the array scanning pattern effectively.
Check Subsequences Using Hashing
For each string, use a hash map to record counts and perform subsequence checks against all other strings. If a string is not a subsequence of any other, it is the longest uncommon subsequence.
Return Result Immediately
Once an uncommon subsequence is found, return its length immediately to avoid unnecessary computation. If none exist after scanning all strings, return -1 to indicate failure.
Complexity Analysis
| Metric | Value |
|---|---|
| Time | Depends on the final approach |
| Space | Depends on the final approach |
Time complexity is O(n^2 * l) where n is the number of strings and l is the maximum string length due to pairwise subsequence checks. Space complexity is O(n * l) for storing strings and hash maps used in the lookup process.
What Interviewers Usually Probe
- Asks if you considered sorting strings by length to optimize finding the longest uncommon subsequence.
- Checks whether you correctly handle duplicates and identical strings affecting subsequence detection.
- Tests understanding of subsequence verification and how to efficiently skip impossible candidates.
Common Pitfalls or Variants
Common pitfalls
- Failing to sort strings, leading to returning a shorter uncommon subsequence instead of the longest.
- Incorrectly identifying a string as uncommon when it is a subsequence of another, especially with duplicates.
- Overlooking edge cases where all strings are identical or nested subsequences exist, resulting in missing the -1 return case.
Follow-up variants
- Consider a version where strings contain uppercase letters or digits requiring adjusted hash checks.
- Allow extremely long strings requiring optimized two-pointer subsequence verification instead of naive scans.
- Find the second-longest uncommon subsequence instead of the longest, changing scan termination logic.
How GhostInterview Helps
- Highlights which strings to scan first using the array length and sorting pattern.
- Automatically performs hash-based subsequence verification across all candidates efficiently.
- Detects edge cases and returns correct results without manual debugging of pairwise checks.
Topic Pages
Related GhostInterview Pages
- LeetCode Interview Copilot - Use GhostInterview as a live solver when you want direct help with LeetCode-style coding questions.
- Coding Interview Assistant - See how GhostInterview supports array, string, linked list, graph, and tree interview workflows.
- How GhostInterview Works - Review the screenshot, reasoning, and answer flow before using the solver in a live interview.
FAQ
What is the primary pattern used in Longest Uncommon Subsequence II?
The main pattern is array scanning combined with hash lookups to verify subsequences efficiently.
How do I handle identical strings in this problem?
Count duplicates with a hash map; a string appearing more than once cannot be an uncommon subsequence.
What is the best approach to check if one string is a subsequence of another?
Use a two-pointer scan comparing characters in order; this ensures correct subsequence verification.
Can Longest Uncommon Subsequence II return -1?
Yes, return -1 if no string is an uncommon subsequence of the others.
How does sorting by string length help solve the problem?
Sorting ensures that the first uncommon subsequence found is the longest, reducing unnecessary comparisons.
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