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Grokking the Coding Interview Patterns in Python

Solution: Number of Wonderful Substrings

Explore how to identify and count wonderful substrings—strings with at most one odd character frequency—by applying hash maps combined with bitmask operations. Understand the step-by-step process of iterating through a string, updating parity with XOR, and leveraging frequency mapping to compute the result efficiently. This lesson helps you grasp a common interview pattern valuable for string manipulation problems and optimized coding solutions.

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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:

  • 1<=1 <= word.length <=103<= 10^3 ...