1759 - Count Number of Homogenous Substrings

Description (leetcode)

Given a string s, return the number of homogenous substrings ofs. 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.

submission

impl Solution {
    pub fn count_homogenous(s: String) -> i32 {
        s.as_bytes()
            .chunk_by(|a, b| a == b)
            .map(|chunk| {
                ((chunk.len() * (chunk.len() + 1) / 2) % 1000000007) as i32
            })
            .sum::<i32>()
            % 1000000007
    }
}