Count Unique Characters of All Substrings of a Given String

Published: Sep 14, 2022

Hard Dynamic Programming Hash Table String


Using hash table, find a repeating character style problems are there. This is the same type. However, this problem asks about all substrings, so how to count the result is not easy. Within non-repeating characters range, we should count substrings.

Problem Description

Let’s define a function countUniqueChars(s) that returns the number of unique characters on s.

  • For example, calling countUniqueChars(s) if s = "LEETCODE" then "L", "T", "C", "O", "D" are the unique characters since they appear only once in s, therefore countUniqueChars(s) = 5.

Given a string s, return the sum of countUniqueChars(t) where t is a substring of s. The test cases are generated such that the answer fits in a 32-bit integer.

Notice that some substrings can be repeated so in this case you have to count the repeated ones too.


  • 1 <= s.length <= 10**5
  • s consists of uppercase English letters only.


Example 1:
Input: s = "ABC"
Output: 10
Example 2
Input: s = "ABA"
Output: 8
Example 3:
Input: s = "LEETCODE"
Output: 92


Start from a hash table which will keep tracking each letter’s start and end indices so far. While going over the given string, add up a count based on the current letter’s previous start and end indices.


class CountUniqueCharactersOfAllSubstringsOfAGivenString:
    def uniqueLetterString(self, s: str) -> int:
        n = len(s)
        memo = {c: (-1, -1) for c in ascii_uppercase}
        result = 0
        for idx, c in enumerate(s):
            i, j = memo[c]
            result += (j - i) * (idx - j)
            memo[c] = (j, idx)
        for c in memo:
            i, j = memo[c]
            result += (n - j) * (j - i)
        return result


  • Time: O(n)
  • Space: O(1)