Number of Wonderful Substrings

Statement

A wonderful string is defined as a string in which at most one letter occurs an odd number of times.

For example:

• The string “aabbc” is wonderful because only the letter 'c' appears an odd number of times.

• The string “pqpq” is wonderful because all letters appear an even number of times.

• The string “mn” is not wonderful because both 'm' and 'n' appear an odd number of times.

You are given a string word that ...