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Grokking the Coding Interview Patterns in Go

Solution: Number of Wonderful Substrings

Explore how to identify and count wonderful substrings—those with at most one odd character count—in a given string using hash maps and bitmasks. Understand step-by-step how to update masks, use frequency maps, and optimize counting substrings with zero or one odd occurrence. This lesson helps you implement a solution with O(NA) time complexity suitable for characters 'a' to 'j'.

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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: ...