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# 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'`.

## ac

```java
```