1759

1759. Count Number of Homogenous Substrings (Medium)

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.

Companies: Virtu Financial

Related Topics:
Math, String

Similar Questions:

• Consecutive Characters (Easy)
• Number of Substrings With Only 1s (Medium)
• Sum of Subarray Ranges (Medium)
• Count the Number of Good Subarrays (Medium)

Hints:

• A string of only 'a's of length k contains k choose 2 homogenous substrings.
• Split the string into substrings where each substring contains only one letter, and apply the formula on each substring's length.

Solution 1.

// OJ: https://leetcode.com/problems/count-number-of-homogenous-substrings
// Author: github.com/lzl124631x
// Time: O(N)
// Space: O(1)
class Solution {
public:
    int countHomogenous(string s) {
        long N = s.size(), cnt = 0, ans = 0, mod = 1e9 + 7;
        for (int i = 0; i < N; ++i) {
            ++cnt;
            if (i == N - 1 || s[i + 1] != s[i]) {
                ans = (ans + cnt * (cnt + 1) / 2 % mod) % mod;
                cnt = 0;
            }
        }
        return ans;
    }
};