Super Pow - Breaking Down the Exponent

Say $b = [1, 2, 3]$, which means $b = 123$. You can write this as $123 = 12 \times 10 + 3$.

So $a^{123} = a^{12 \times 10 + 3} = (a^{12})^{10} \times a^3$. This breaks the problem into two smaller exponents: $a^{12}$ raised to the 10th power, and $a^3$.

You can recurse on the left part ($[1, 2]$) and handle the last digit ($3$) separately. Each step reduces the size of $b$ by one digit.