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Grokking the Coding Interview Patterns in Python

Solution: Number of Wonderful Substrings

Explore how to determine the number of wonderful substrings in a string, defined as substrings with at most one letter appearing an odd number of times. This lesson teaches the use of hash maps and bitmasks to track character parity efficiently, helping you solve substring frequency problems with optimized time and space complexity.

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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: ...