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

Constraints:

1 <= s.length <= 105
s consists of lowercase letters.

Solution:

for n consecutive chars, c, cc, ccc, c...cc (n c's) appear 1 + 2 + 3 + ... + n times

class Solution {
    int mod = (int) (1e9) + 7;
    
    public int countHomogenous(String s) {
        int n = s.length();
        int[] sum = calc(n);
        int count = 1;
        int res = 0;
        char prev = s.charAt(0);
        for (int i = 1; i < n; i ++) {
            char curr = s.charAt(i);
            if (prev == curr) {
                count ++;
            } else {
                // System.out.println(prev + ", " + count + ", " + sum[count]);
                res = (res + sum[count]) % mod;
                count = 1;
            }
            prev = curr;
        }
        res = (res + sum[count]) % mod;
        return res;
    }
    
    public int[] calc(int n) {
        int[] res = new int[n + 1];
        res[0] = 0;
        for (int i = 1; i <= n; i ++) {
            res[i] = (res[i - 1] + i) % mod;
        }
        return res;
    }
}