Compute letter frequencies from encoded string

Q: Compute letter frequencies from encoded string

This question evaluates proficiency in string parsing and frequency aggregation, specifically handling mixed-length encodings, tokenization of digits and '#' markers, and parsing numeric repetition counts while respecting linear-time and constant-space constraints.

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Coding & Algorithms questions require understanding of core concepts and practice. PracHub provides solutions with explanations to help you master coding & algorithms interviews.

Q: What difficulty level is this interview question?

This is a medium difficulty Coding & Algorithms question, commonly asked during Technical Screen rounds at Oracle.

Q: What role is this question designed for?

This question is commonly asked for Software Engineer candidates at Oracle during technical interviews.

Question

Problem

You are given an encoded string s representing a string of lowercase English letters (a–z). The encoding follows these rules:

Letters a to i are encoded as digits 1 to 9 .
- a -> "1" , b -> "2" , …, i -> "9"
Letters j to z are encoded as two digits followed by # .
- j -> "10#" , k -> "11#" , …, z -> "26#"
If a letter repeats consecutively k >= 2 times, the repetition is encoded by appending "(k)" immediately after that letter’s code.
- Example: "aa" -> "1(2)" , "ccc" -> "3(3)" , "jj" -> "10#(2)"

Task

Return an integer array counts of length 26, where counts[0] is the number of 'a' characters in the decoded string, counts[1] is the number of 'b', …, and counts[25] is the number of 'z'.

Examples

s = "1226#24#" decodes to "abzx" .
s = "1(2)23(3)" decodes to "aabccc" .
s = "2110#(2)" decodes to "bajj" .
s = "23#(2)24#25#26#23#(3)" decodes to "wwxyzwww" .

Input/Output

Input: a string s consisting of digits, # , ( , and ) that follows the encoding rules above.
Output: an array of 26 integers with letter frequencies.

Notes / Constraints (reasonable interview assumptions)

s is validly encoded.
Counts in parentheses can be multiple digits (e.g., (12) ).
Aim for O(|s|) time and O(1) extra space (excluding the output array).