Leetcode 3084. Count Substrings Starting and Ending with Given Character

• [Leetcode 3084. Count Substrings Starting and Ending with Given Character](#Leetcode 3084. Count Substrings Starting and Ending with Given Character)
  • [1. 解题思路](#1. 解题思路)
  • [2. 代码实现](#2. 代码实现)

• 题目链接：3084. Count Substrings Starting and Ending with Given Character

1. 解题思路

这一题其实挺简单的，只要看一下目标的character在string当中出现了几次即可，然后就是一个 C n 2 C_n^2 Cn2的头尾选择问题即可。

2. 代码实现

给出python代码实现如下：

class Solution:
    def countSubstrings(self, s: str, c: str) -> int:
        cnt = Counter(s)[c]
        return cnt * (cnt+1) // 2

提交代码评测得到：耗时52ms，占用内存17.5MB。