Skip to content

1759. Count Number of Homogenous Substrings

Difficulty Topics

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.

Solution

count-number-of-homogenous-substrings.py
class Solution:
    def countHomogenous(self, s: str) -> int:
        n = len(s)
        M = 10 ** 9 + 7
        res = 0

        mp = Counter(s)

        i = 0
        while i < n:
            j = i + 1
            while j < n and s[i] == s[j]:
                res += j - i
                j += 1

            i = j

        return (res + sum(mp.values())) % M