CodingHardcod-g416

Subject MatrixLevel Senior–Staff~35 minCommon in Algorithms & data structures interviewsIndustries Software development

Question

Count the number of distinct strings of length n over the alphabet {a, b, c} such that no two adjacent characters are equal AND the string never contains 'a' immediately followed by 'b'. Because n can be as large as 10^18, you must use matrix exponentiation; return the count modulo 1000000007. For n = 0 there is exactly one string (the empty string).

Implement

count_strings(n: int) → int

Examples

in[1]out3

What a strong answer looks like

State your approach and its time/space complexity out loud before you optimize. Handle the edge cases (empty input, duplicates, overflow), and say why you chose this over the brute force. Green tests are the floor, not the grade.

Learn the concepts

Vibe coding: describe the solution in plain language (or narrate it) and the coach grades your approach. Generating runnable code from your description is coming next.

Run or narrate your approach, then ask the coach.