Grand Digit Total — Solution & Editorial

Back to the Problem

Per position: full cycles contribute 0+1+...+9 = 45 per block of 10p; the partial cycle adds the triangular count of the current digit plus its tail. Sum over positions.

Complexity: O(log N)

Watch out: The total is ~8.1×10¹⁹ at the limit... which exceeds 64 bits! Use unsigned 64 carefully or 128-bit/big integers.