CodingHardcod-g725

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

Question

Count integers x in [1, N] (decimal, no leading zeros) such that no two ADJACENT digits of x are equal. Return the count. Constraints: 1 <= N <= 10**18. A direct loop is infeasible; use digit DP tracking the previous digit, a tight flag, and a started flag (to handle leading zeros / the length).

Implement

count_no_adjacent_equal(N: int) → int

Examples

in[12]out11

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.