CountNumberOfHomogenousSubstrings.cpp

// Source : https://leetcode.com/problems/count-number-of-homogenous-substrings/
// Author : Hao Chen
// Date   : 2021-02-14

/***************************************************************************************************** 
 *
 * 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.
 ******************************************************************************************************/

class Solution {
public:
    int countHomogenous(string s) {
        long long result = 0;
        char current = '\0';
        long long len = 0;
        for(int i = 0; i < s.size() ; i++) {
            if (current != s[i] ){
                //sum from 1 to len
                result += len * (len+1)/2;
                current = s[i];
                len = 1;
            }else{
                len++;
            }
        }
        //the rest string
        result += len * (len+1)/2;
        return result % 1000000007;
    }
};