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 consists of lowercase English letters from 'a' to 'j'. Your task is to return the total number of wonderful non-empty substringsA substring is any contiguous sequence of characters within the string. in word.

Note: If a substring appears multiple times, each occurrence should be counted separately.

Constraints:

• $1 <=$ word.length $<= 10^3$

• word consists of lowercase English letters from 'a' to 'j'.

The solution uses a hash map to count the number of wonderful substrings in a given string word. A wonderful substring is defined as a string in which at most one letter appears an odd number of times. The approach leverages a bitmask to track the parity of character frequencies efficiently.

Let’s go through the steps of the solution:

1. We create a hash map called freq to store the frequency of each bitmask. Each key in this hash map represents a bitmask that corresponds to the characters in the substring, while the value represents how many times that bitmask has been encountered.

2. We initialize freq[0] = 1, where $0$ represents an empty prefix, which serves as a base ...