Stepping Numbers - Implementation

When $started = false$: placing 0 keeps $started = false$. Placing 1-9 sets $started = true$ and $lastDigit$. When $started = true$: only place digits within 1 of $lastDigit$. Update $lastDigit$ for next position. Edge case: what's the first digit of 0? We define single-digit numbers as valid (no adjacent pair to check). Time: $O(n \cdot 2 \cdot 10 \cdot 2) = O(n)$ where $n = \log_{10} N$. The constants are small. Space: $O(\log N \cdot 10)$ for memoization table.

Time complexity: $O(\log N \cdot 10)$.

Space complexity: $O(\log N)$.