creating a secret code

Description

The Problem Statement
You and your roommate are creating a secret code.

You have a rather simple scheme to express strings of letters as numbers.
You realize that since there are only 26 different letters, a string of letters can be viewed as a number in base 26!
For example, consider the word:

COMPUTER

You realize that we can just assign numeric values to the letters with A = 0, B = 1, C = 2, …, Z = 25 and then treat each letter location as the ones place, the 26 place, the 262 place and so forth.
Thus, the value of COMPUTER would be:

2×26^7 + 14×26^6 + 12×26^5 + 15×26^4 + 20×26^3 + 19×26^2 + 4×26^1 + 17×26^0.

Fill in the two functions to encode and decode your messages.
One function takes in a string and its length, and returns a number (stored in a data type called long long) equivalent to the string using the system described above.
The second function will take in a number and the length of the corresponding word and print out the corresponding string that is the result of decoding the first number.