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 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

/**
 * @param {string} s
 * @return {number}
 */
var countHomogenous = function(s) {
    var startIndex = 0;
    var sum = 0;
    var mod = Math.pow(10, 9) + 7;
    for (var i = 0; i < s.length; i++) {
        if (s[i] === s[i + 1]) continue;
        var num = i - startIndex + 1;
        sum += ((1 + num) * num / 2) % mod;
        sum %= mod;
        startIndex = i + 1;
    }
    return sum;
};

Explain:

nope.

Complexity:

• Time complexity : O(n).
• Space complexity : O(1).