Problem - Super Pow

Compute $a^b \mod 1337$ where $a$ is an integer and $b$ is a large number represented as an array of digits.

For example, $a = 2$ and $b = [1, 0]$ means $b = 10$. You need $2^{10} \mod 1337 = 1024$.

The array $b$ can have thousands of digits, so you can't convert it to an integer directly. You need a way to compute the exponent piece by piece.