21. Count Number of Homogenous Substrings

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

Solution:

Approach: No of substring of a string n:- (Σn = n * (n+1)/2 ) We would find the length of homogeneous substring and count no ways possible

class Solution
{
public:
    int countHomogenous(string s)
    {

        int mod = 1000000007;
        long long int count = 0;

        int n = s.length();

        int i = 0;
        int j = 1;

        while (i < n)
        {

            while (s[i] == s[j] && j < n)
            {
                j++;
            }

            long long int len = j - i;
            long long int ways = (len * (len + 1)) / 2;
            count += ways;
            i = j;
            j++;
        }

        return count % mod;
    }
};

Time Complexity: O(n)