Given three numbers a, b and m. The task is to find (a^b)%m.
Note: a is given as a String because it can be very big.

Example 1:
Input:
a = "3", b = 2, m = 4
Output:
1
Explanation:
(3^2)%4 = 9%4 = 1
Example 2:
Input:
a = "8", b = 2, m = 6
Output:
4
Explanation:
(8^2)%6 = 64%6 = 4

Your Task:
You don't need to read input or print anything. Your task is to complete the function powerMod() which takes a string a and 2 integers b, and m as input and returns (a^{b} % m).

Expected Time Complexity: O(Number of digits in a)
Expected Auxiliary Space: O(1)

Constraints:
1 <= |a| <= 10^{5}
0 <= b <= 10^{9}
1 <= m <=10^{9}

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