Substrings That Begin and End With the Same Letter: coding question

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Count the number of substrings of a given string whose first and last character are the same (including single-character substrings). The challenge is to do this efficiently for long strings by aggregating counts per character and using combinatorics to avoid O(n^2) enumeration.

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Leetcode 2083. Substrings That Begin and End With the Same Letter
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Substrings That Begin and End With the Same Letter

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Substrings That Begin and End With the Same Letter

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