Is Count Substrings With K-Frequency Characters I easy or hard?

The problem is rated Medium because it requires recognizing a sliding window counting pattern rather than brute force substring enumeration. Once the key observation about counting n - right substrings is understood, the implementation becomes straightforward.

Count Substrings With K-Frequency Characters I Python/Java solution

The typical implementation uses a sliding window with a dictionary (Python) or HashMap/array (Java). Track character frequencies, expand the right pointer, and when a frequency reaches k, count all remaining valid substrings. The same logic easily translates to C++, JavaScript, and other languages.

How to solve Count Substrings With K-Frequency Characters I in O(n)?

Maintain a sliding window with two pointers and a frequency map. Expand the right pointer and update counts. Once any character frequency reaches k, all substrings ending at or after that position are valid, so add n - right to the answer and shrink the window from the left.

What is the best approach for Count Substrings With K-Frequency Characters I?

The most efficient approach uses a sliding window with a hash map to track character frequencies. When a character count reaches k, every extension of that window to the right remains valid, so you can add multiple substrings at once. This technique reduces the complexity to O(n) time with O(26) space.

Is Count Substrings With K-Frequency Characters I asked at Google/Amazon/Meta?

Sliding window and substring frequency problems frequently appear in interviews at companies like Amazon, Google, and Meta. Variations of this question test understanding of two pointers, hash maps, and efficient substring counting.

What data structure is used in Count Substrings With K-Frequency Characters I?

The main data structure is a hash map or fixed-size frequency array used to count character occurrences in the current window. It works together with the sliding window technique to update counts in constant time while moving pointers.

What is the time complexity of Count Substrings With K-Frequency Characters I?

The optimal solution runs in O(n) time using a sliding window and frequency map. Each character is added and removed from the window at most once. A brute force approach that checks all substrings takes O(n^2 * 26) time.

Description Hints 3 Solution Video 0:48

3325. Count Substrings With K-Frequency Characters I

Medium56.3% acceptance1 company

Given a string s and an integer k, return the total number of substrings of s where at least one character appears at least k times.

Example 1:

Input: s = "abacb", k = 2

Output: 4

Explanation:

The valid substrings are:

"aba" (character 'a' appears 2 times).
"abac" (character 'a' appears 2 times).
"abacb" (character 'a' appears 2 times).
"bacb" (character 'b' appears 2 times).

Example 2:

Input: s = "abcde", k = 1

Output: 15

Explanation:

All substrings are valid because every character appears at least once.

Constraints:

1 <= s.length <= 3000
1 <= k <= s.length
s consists only of lowercase English letters.

Asked at 1 company

Google

Topics

Hints open one at a time

Hint 1

Fix the left index of the substring.

Hint 2

For the fixed left index, find the first right index for which substring s[left..right] satisfies the condition.

Hint 3

Every substring that starts at left and ends after right satisfies the condition.

0:48

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1class Solution:
2    def numberOfSubstrings(self, s: str, k: int) -> int:
3        

Starter codePython 3Spaces: 4

Examples

Case 1Case 2

Input

"abacb"
2

Input

"abcde"
1

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