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Grokking the Coding Interview Patterns

Solution: Number of Wonderful Substrings

Understand how to identify and count wonderful substrings in a string where at most one letter appears an odd number of times. Explore the use of hash maps and bitmasking to efficiently track character parity, and learn to calculate the total number of such substrings using a practical algorithm with clear time and space complexity analysis.

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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: ...