Digits on the Pages — Solution & Editorial
Group by digit-length: the d-digit numbers span [10^(d−1), 10^d−1]; each contributes d digits. Sum the clipped blocks.
Complexity: O(log N)
Group by digit-length: the d-digit numbers span [10^(d−1), 10^d−1]; each contributes d digits. Sum the clipped blocks.
Complexity: O(log N)