1915 - Number of Wonderful Substrings.

Medium

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:

• "aba" -> "a"

• "aba" -> "b"

• "aba" -> "a"

• "aba" -> "aba"

Example 2:

Input: word = "aabb"

Output: 9

Explanation: The nine wonderful substrings are underlined below:

• "aabb" -> "a"

• "aabb" -> "aa"

• "aabb" -> "aab"

• "aabb" -> "aabb"

• "aabb" -> "a"

• "aabb" -> "abb"

• "aabb" -> "b"

• "aabb" -> "bb"

• "aabb" -> "b"

Example 3:

Input: word = "he"

Output: 2

Explanation: The two wonderful substrings are underlined below:

• "he" -> "h"

• "he" -> "e"

Constraints:

• <code>1 <= word.length <= 10⁵</code>

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