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Grokking the Coding Interview Patterns

Solution: Number of Wonderful Substrings

Explore how to count the total number of wonderful substrings within a string, defined by having at most one character with an odd frequency. Understand how to use a hash map combined with bitmasking to efficiently track parity of characters and solve the problem using optimized iteration and frequency tracking methods.

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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 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: ...