Count Number of Homogenous Substrings — LeetCode 1759 Python Solution

The problem

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

Example

Input

s = "abbcccaa"

Output

13

Explanation

The homogenous substrings are listed as below:

Python solution

class Solution:
    def countHomogenous(self, s: str) -> int:
        mod = 10**9 + 7
        i, n = 0, len(s)
        ans = 0
        while i < n:
            j = i
            while j < n and s[j] == s[i]:
                j += 1
            cnt = j - i
            ans += (1 + cnt) * cnt // 2
            ans %= mod
            i = j
        return ans

Complexity

Pattern: Math and Number Theory

Find the closed form, the invariant, or the modular identity — and skip the loop entirely. LeetCode 1759. Count Number of Homogenous Substrings is filed here on both counts: the reference solution below belongs to the algorithm family this hub collects, and LeetCode tags it Math.

The math and number theory guide has the Python template for the pattern and the 485 LeetCode problems that use it.

Related problems