Leetcode : 521. Longest Uncommon Subsequence I

Problem Statement

Given two strings a and b, return the length of the **longest uncommon subsequence* between* a and b. If the longest uncommon subsequence does not exist, return -1.

An uncommon subsequence between two strings is a string that is a subsequence of one but not the other.

A subsequence of a string s is a string that can be obtained after deleting any number of characters from s.

For example, "abc" is a subsequence of "aebdc" because you can delete the underlined characters in "aebdc" to get "abc". Other subsequences of "aebdc" include "aebdc", "aeb", and "" (empty string).

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.

Constraints:

1 <= a.length, b.length <= 100

a and b consist of lower-case English letters.

Idea

Brute Force Solution: Generate all the subsequences of both the strings and store the frequency of each subsequence in a hashmap/dictionary. The subsequence whose frequency equal to 1 will be the longest uncommon subsequence. If there is not subsequence with frequency as 1 , then there is no uncommon subsequence for given strings.

Optimized Solution:

If both the given strings are equal then there will not be uncommon subsequences for given strings. In this case we can directly return -1
If the strings are not equal and length of both the strings are equal , then one string cannot be the subsequence of other string. In this case we can either return length of any one of the strings.
If both strings are not equal and length of one string is greater than length of other string, then in this case we can directly return the length of the largest string as the largest string cannot be the subsequence of the smaller string.

Code(Python)

class Solution:
    def findLUSlength(self, a: str, b: str) -> int:
        if a == b:
            return -1
        a_length = len(a)
        b_length = len(b)
        return max(a_length,b_length)