3084. Count Substrings Starting and Ending with Given Character

Problem Link

Description

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You are given a string s and a character c. Return the total number of substrings of s that start and end with c.

 

Example 1:

Input: s = "abada", c = "a"

Output: 6

Explanation: Substrings starting and ending with "a" are: "abada", "abada", "abada", "abada", "abada", "abada".

Example 2:

Input: s = "zzz", c = "z"

Output: 6

Explanation: There are a total of 6 substrings in s and all start and end with "z".

 

Constraints:

• 1 <= s.length <= 105
• s and c consist only of lowercase English letters.

Solution

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Python3

class Solution:
    def countSubstrings(self, s: str, c: str) -> int:
        N = len(s)
        count = res = 0
 
        for x in s:
            if x == c:
                count += 1
                res += count
        
        return res