Is Longest Uncommon Subsequence I easy or hard?

LeetCode classifies this problem as Easy. The challenge comes from recognizing the key observation that identical strings eliminate all uncommon subsequences. Once that insight is clear, the implementation becomes a simple comparison.

Longest Uncommon Subsequence I Python/Java solution

In Python or Java, check if the two strings are equal. If they are equal, return -1. Otherwise return max(len(a), len(b)) in Python or Math.max(a.length(), b.length()) in Java. This concise logic implements the optimal O(n) solution.

How to solve Longest Uncommon Subsequence I in O(n)?

Compare the two input strings directly. If they are identical, return -1 because every subsequence will match in both strings. If they differ, return the maximum of their lengths since the longer string itself cannot be a subsequence of the shorter one. This solution scans the strings once, giving O(n) time complexity.

What is the best approach for Longest Uncommon Subsequence I?

Direct string comparison is the optimal approach. If the two strings are equal, return -1 because every subsequence appears in both. Otherwise, the longer string itself is the longest uncommon subsequence. This runs in O(n) time with O(1) space.

Is Longest Uncommon Subsequence I asked at Google/Amazon/Meta?

Variants of subsequence and string comparison problems frequently appear in interviews at companies like Amazon, Google, and Meta. While this exact question is easier than typical interview problems, it tests understanding of subsequences and recognizing hidden simplifications.

What data structure is used in Longest Uncommon Subsequence I?

The problem mainly uses basic string operations. Some implementations demonstrate subsequence checks using the two-pointer technique on strings, but the optimal solution only requires direct string comparison and length evaluation.

What is the time complexity of Longest Uncommon Subsequence I?

The optimal solution runs in O(n) time where n is the length of the longer string, because it only requires a string equality check and length comparison. Space complexity is O(1) since no additional data structures are required.

Description Hints 3 Solution Video 5:38

521. Longest Uncommon Subsequence I

Easy62.7% acceptance2 companies

Given two strings a and b, return the length of the longest uncommon subsequence between a and b. If no such uncommon subsequence exists, return -1.

An uncommon subsequence between two strings is a string that is a subsequence of exactly one of them.

Example 1:

Input: a = "aba", b = "cdc"
Output: 3
Explanation: One longest uncommon subsequence is "aba" because "aba" is a subsequence of "aba" but not "cdc".
Note that "cdc" is also a longest uncommon subsequence.

Example 2:

Input: a = "aaa", b = "bbb"
Output: 3
Explanation: The longest uncommon subsequences are "aaa" and "bbb".

Example 3:

Input: a = "aaa", b = "aaa"
Output: -1
Explanation: Every subsequence of string a is also a subsequence of string b. Similarly, every subsequence of string b is also a subsequence of string a. So the answer would be -1.

Constraints: