372. Super Pow 
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Problem
Your task is to calculate ab mod 1337 where a is a positive integer and b is an extremely large positive integer given in the form of an array.
Example 1:
Input: a = 2, b = [3] Output: 8
Example 2:
Input: a = 2, b = [1,0] Output: 1024
Example 3:
Input: a = 1, b = [4,3,3,8,5,2] Output: 1
Constraints:
1 <= a <= 231 - 11 <= b.length <= 20000 <= b[i] <= 9bdoes not contain leading zeros.