count-number-of-homogenous-substrings

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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.

Related Topics

[Greedy] [String]

Hints

Hint 1

A string of only 'a's of length k contains k choose 2 homogenous substrings.

Hint 2

Split the string into substrings where each substring contains only one letter, and apply the formula on each substring's length.