Super Pow

For the common Super Pow problem, the exponent is an array of decimal digits and the modulus is fixed, so never build the full exponent. Process digits left to right with the identity: if the prior exponent is E and the next digit is d, then a^(10E+d) = (a^E)^10 * a^d. Maintain result, initially one. For each digit, set result = modPow(result,10) * modPow(a,d) mod M. Implement modPow(base, exponent) by binary exponentiation, reducing after each multiplication. This works for arbitrarily long exponent digit arrays because state remains bounded. With a constant modulus and digit exponent at most nine, time is O(number of digits * log 10) plus constant-width modular operations, and space is O(1) aside from input. Clarify the modulus and whether a must first be reduced; it should. Avoid relying on floating-point pow, which loses precision.