Hash Divided String

Hash Divided String

What is this problem about?

The Hash Divided String coding problem is a string transformation task. You are given a string $s$ of length $n$ and an integer $k$, where $n$ is a multiple of $k$. Your task is to divide the string into substrings of length $k$. For each substring, calculate a "hash" value by summing the positions of its characters in the alphabet (where 'a' is 0, 'b' is 1, ..., 'z' is 25). Then, take the sum modulo 26 and convert it back to a character. The final result is the concatenation of these hashed characters.

Why is this asked in interviews?

Google uses simulation problems like this to test a candidate's ability to follow complex procedural instructions and handle character-to-integer mappings. It evaluates your proficiency with Simulation interview patterns and string slicing. It also checks if you can handle modular arithmetic correctly, which is vital for many hashing and encryption algorithms.

Algorithmic pattern used

This problem uses a Simulation pattern with Linear Scan.

1. Iterate through the string in chunks of size $k$.
2. For each chunk, use a nested loop to sum the ASCII-based values of the characters.
3. Apply the modulo 26 operation.
4. Map the result back to a character (usually by adding the value back to the ASCII value of 'a').
5. Append to a result string.

Example explanation

String: "abcd", $k=2$.

1. Divide into ["ab", "cd"].
2. First chunk "ab": 'a' $o$ 0, 'b' $o$ 1. Sum = $0 + 1 = 1$. $1 \pmod{26} = 1$. Character 'b'.
3. Second chunk "cd": 'c' $o$ 2, 'd' $o$ 3. Sum = $2 + 3 = 5$. $5 \pmod{26} = 5$. Character 'f'. Result: "bf".

Common mistakes candidates make

• Incorrect Alphabet Mapping: Thinking 'a' is 1 instead of 0, which shifts every character in the result.
• String Slicing errors: Miscalculating the boundaries of the substrings during iteration.
• Inefficient Concatenation: Repeatedly creating new string objects in a loop instead of using a StringBuilder (Java) or joining a list (Python).

Interview preparation tip

When a problem involves mapping 'a-z' to 0-25, use the expression (char - 'a') to get the integer value and (char)(value + 'a') to convert it back. This is more robust than using a hardcoded Hash Map of 26 characters.