1915. Number of Wonderful Substrings

https://leetcode.com/problems/number-of-wonderful-substrings

Description

A wonderful string is a string where at most one letter appears an odd number of times.

• For example, "ccjjc" and "abab" are wonderful, but "ab" is not.

Given a string word that consists of the first ten lowercase English letters ('a' through 'j'), return the number of wonderful non-empty substrings in word. If the same substring appears multiple times in word, then count each occurrence separately.

A substring is a contiguous sequence of characters in a string.

Example 1:

**Input:** word = "aba"
**Output:** 4
**Explanation:** The four wonderful substrings are underlined below:
- "**a**ba" -> "a"
- "a**b**a" -> "b"
- "ab**a**" -> "a"
- "**aba**" -> "aba"

Example 2:

**Input:** word = "aabb"
**Output:** 9
**Explanation:** The nine wonderful substrings are underlined below:
- "**a**abb" -> "a"
- "**aa**bb" -> "aa"
- "**aab**b" -> "aab"
- "**aabb**" -> "aabb"
- "a**a**bb" -> "a"
- "a**abb**" -> "abb"
- "aa**b**b" -> "b"
- "aa**bb**" -> "bb"
- "aab**b**" -> "b"

Example 3:

**Input:** word = "he"
**Output:** 2
**Explanation:** The two wonderful substrings are underlined below:
- "**h**e" -> "h"
- "h**e**" -> "e"

Constraints:

• 1 <= word.length <= 105

• word consists of lowercase English letters from 'a' to 'j'.

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