1759. Count Number of Homogenous Substrings

T: O(n)
S: O(1)

```java
class Solution {
    public int countHomogenous(String s) {
        int mod = 1000000007;
        long count = 0;
        long result = 0;
        char pre = '0';
        for (int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);
            if (c != pre) {
                count = 1;
                pre = c;
            } else {
                count++;
            }
            result += count;
        }
        return (int)(result % mod);
    }
}

/***
subarry -> homogenous -> 

這題就分開來計算, 看subarray 是 homogenous 就可以了, 算這個 subarray 能產生的結果總數 (ex: ccc -> 6 種可能, c 3種, cc 2種, ccc 1種 = 3+2+1)
a = 1
bb = 3
ccc = 6
aa = 3
so 1+3+6+3 = 13

ccc = 3+2+1
bb = 2+1
a = 1


zzzzz = 
5+4+3+2+1

so just accumulate cuurent count (for this char)



T: O(n)
S: O(1)
 */
```