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Grokking the Coding Interview Patterns

Solution: Number of Wonderful Substrings

Explore how to identify and count wonderful substrings, defined by at most one letter appearing an odd number of times, using hash maps and bitmasks. Understand the step-by-step approach to track character frequencies efficiently, analyze time and space complexity, and implement this solution in coding interviews involving strings restricted to 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:

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