Special Integers - Walkthrough

Count numbers $\leq 135$ with all distinct digits. Use a bitmask to track used digits. Start: mask = 0, tight = true, started = false. Position $0$: try $0$ (leading zero, don't add to mask), 1 (mask |= 2).

If we chose 1 (mask = 0b10, tight = true): position $1$ can be $0$, $2$, $3$ (not 1, it's used). If we choose $3$ (> limit's 3), invalid. Continue to position $2$. Count paths where each digit appears at most once. Bitmask tracks usage in $O(1)$ per transition.