1131. Count Substrings with Only One Distinct Letter

• Difficulty: Easy

• Topics: Math, String

• Similar Questions:

Problem:

Given a string S, return the number of substrings that have only one distinct letter.

Example 1:

Input: S = "aaaba"
Output: 8
Explanation: The substrings with one distinct letter are "aaa", "aa", "a", "b".
"aaa" occurs 1 time.
"aa" occurs 2 times.
"a" occurs 4 times.
"b" occurs 1 time.
So the answer is 1 + 2 + 4 + 1 = 8.

Example 2:

Input: S = "aaaaaaaaaa"
Output: 55

Constraints:

• 1 <= S.length <= 1000
• S[i] consists of only lowercase English letters.

Solutions:

class Solution {
public:
    int countLetters(string S) {
        int count = 0;
        int consecutive = 1;
        for (int i = 1; i < S.length(); ++i) {
            if (S[i] == S[i-1]) {
                ++consecutive;
            } else {
                count += calculate(consecutive);
                consecutive = 1;
            }
        }

        count += calculate(consecutive);
        return count;
    }

private:
    int calculate(int num) {
        return num * (num + 1) / 2;
    }

};