Is Number of Equal Count Substrings easy or hard?

Number of Equal Count Substrings is typically rated Medium difficulty. The challenge is recognizing that valid substrings must have length k * count and using a sliding window with frequency counting to avoid quadratic enumeration.

Number of Equal Count Substrings Python/Java solution

Implement a sliding window for each possible number of distinct characters. Maintain a frequency array, update counts when the window moves, and check whether exactly k characters appear count times. The same logic translates easily across Python, Java, C++, Go, and JavaScript.

How to solve Number of Equal Count Substrings in O(n)?

Strictly O(n) is not typical because the algorithm enumerates possible distinct character counts. However, since the alphabet size is fixed at 26, the complexity becomes O(26 * n), which is effectively linear. A sliding window of size k * count is maintained for each possible k.

What is the best approach for Number of Equal Count Substrings?

The most efficient method is enumeration combined with a sliding window. Iterate over the possible number of distinct characters k (1–26), fix the window length to k * count, and maintain character frequencies while sliding the window. This avoids checking all substrings and runs in O(26 * n) time with O(26) space.

Is Number of Equal Count Substrings asked at Google/Amazon/Meta?

Problems involving substring frequency constraints and sliding window counting patterns frequently appear in interviews at companies like Google, Amazon, and Meta. Variants of this problem test understanding of string frequency tracking and window-based optimization.

What data structure is used in Number of Equal Count Substrings?

The core data structure is a fixed-size frequency array (size 26 for lowercase letters). It tracks how many times each character appears inside the current sliding window, enabling constant-time updates while expanding or shrinking the window.

What is the time complexity of Number of Equal Count Substrings?

The optimal solution runs in O(26 * n) time because the algorithm scans the string once for each possible number of distinct characters (at most 26). Space complexity is O(26) for the character frequency array used inside the sliding window.

Description Hints 4 Solution Video 10:56

2067. Number of Equal Count Substrings

Medium45.1% acceptance1 companyPremium elsewhere, free here

You are given a 0-indexed string s consisting of only lowercase English letters, and an integer count. A substring of s is said to be an equal count substring if, for each unique letter in the substring, it appears exactly count times in the substring.

Return the number of equal count substrings in s.

A substring is a contiguous non-empty sequence of characters within a string.

Example 1:

Input: s = "aaabcbbcc", count = 3
Output: 3
Explanation:
The substring that starts at index 0 and ends at index 2 is "aaa".
The letter 'a' in the substring appears exactly 3 times.
The substring that starts at index 3 and ends at index 8 is "bcbbcc".
The letters 'b' and 'c' in the substring appear exactly 3 times.
The substring that starts at index 0 and ends at index 8 is "aaabcbbcc".
The letters 'a', 'b', and 'c' in the substring appear exactly 3 times.

Example 2:

Input: s = "abcd", count = 2
Output: 0
Explanation:
The number of times each letter appears in s is less than count.
Therefore, no substrings in s are equal count substrings, so return 0.

Example 3:

Input: s = "a", count = 5
Output: 0
Explanation:
The number of times each letter appears in s is less than count.
Therefore, no substrings in s are equal count substrings, so return 0

Constraints:

1 <= s.length <= 3 * 10⁴
1 <= count <= 3 * 10⁴
s consists only of lowercase English letters.

Asked at 1 company

Cisco

Topics

Hints open one at a time

Hint 1

The brute force solution is to check every substring, which would TLE. How can we improve this solution?

Hint 2

In an equal count substring, the first character appears count times, the second character appears count times, and so on.

Hint 3

The length of an equal count substring is the number of unique characters multiplied by count.

Hint 4

The length of all equal count substrings are multiples of count.

10:56

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