1759. Count Number of Homogenous Substrings

Description

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.

 

Solutions

Solution: Math

• Time complexity: O(n)
• Space complexity: O(1)

 

JavaScript

/**
 * @param {string} s
 * @return {number}
 */
const countHomogenous = function (s) {
  const MODULO = 10 ** 9 + 7;
  let result = (count = 0);

  for (let index = 0; index <= s.length; index++) {
    if (!index || s[index] === s[index - 1]) {
      count += 1;
      continue;
    }
    result += ((count + 1) * count) / 2;
    result %= MODULO;
    count = 1;
  }
  return result;
};