1759. Count Number of Homogenous Substrings

Problem

Given a string s, return **the number of *homogenous* substrings of s.** Since the answer may be too large, return it modulo 10^9 + 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 <= 10^5

• s consists of lowercase letters.

Solution (Java)

class Solution {
    public int countHomogenous(String s) {
        int total = 0;
        int count = 0;
        for (int i = 0; i < s.length(); i++) {
            if (i > 0 && s.charAt(i) == s.charAt(i - 1)) {
                count++;
            } else {
                count = 1;
            }
            total = (total + count) % 1000000007;
        }
        return total;
    }
}

Explain:

nope.

Complexity:

• Time complexity : O(n).
• Space complexity : O(n).