Given a string `s`, return the number of homogenous substrings of `s`. Since the answer may be too large, return it modulo `109 + 7`.

A string is homogenous if all the characters of the string are the same.

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

Example 1:

    Input: s = "abbcccaa"

    Output: 13

    Explanation: The homogenous substrings are listed as below:

    "a"   appears 3 times.

    "aa"  appears 1 time.

    "b"   appears 2 times.

    "bb"  appears 1 time.

    "c"   appears 3 times.

    "cc"  appears 2 times.

    "ccc" appears 1 time.

    3 + 1 + 2 + 1 + 3 + 2 + 1 = 13.

Example 2:

    Input: s = "xy"

    Output: 2

    Explanation: The homogenous substrings are "x" and "y".

Example 3:

    Input: s = "zzzzz"

    Output: 15

Constraints:

- `1 <= s.length <= 105`
- `s` consists of lowercase letters.