Is Count Substrings That Satisfy K-Constraint I easy or hard?

The problem is considered Easy on LeetCode because it applies a standard sliding window pattern. Once you recognize that the substring becomes invalid only when both counts exceed k, the implementation becomes straightforward.

Count Substrings That Satisfy K-Constraint I Python/Java solution

Most implementations use the same sliding window logic in any language. Maintain left and right pointers, track counts of '0' and '1', shrink the window when both exceed k, and add (right - left + 1) to the answer. This works in Python, Java, C++, and JavaScript with O(n) time complexity.

How to solve Count Substrings That Satisfy K-Constraint I in O(n)?

Use a sliding window with two pointers. Expand the right pointer while tracking counts of 0s and 1s. If both counts become greater than k, move the left pointer forward until at least one count is ≤ k again. For each right index, add (right - left + 1) to the result to count all valid substrings ending at that position.

What is the best approach for Count Substrings That Satisfy K-Constraint I?

The sliding window (two pointers) approach is the most efficient. It maintains a window with counts of zeros and ones and shrinks the window only when both counts exceed k. Each index is processed at most twice, resulting in O(n) time and O(1) space.

Is Count Substrings That Satisfy K-Constraint I asked at Google/Amazon/Meta?

Problems involving substring counting with sliding window patterns frequently appear in interviews at companies like Amazon, Google, and Meta. Variants that track character frequencies or maintain constraints inside a window are common interview exercises.

What data structure is used in Count Substrings That Satisfy K-Constraint I?

The solution primarily uses the sliding window technique with two pointers and simple integer counters to track the number of zeros and ones. No advanced data structures are required beyond basic variables.

What is the time complexity of Count Substrings That Satisfy K-Constraint I?

The optimal sliding window solution runs in O(n) time because each character is added and removed from the window at most once. A straightforward brute force approach that checks all substrings takes O(n²) time.

Description Hints 1 Solution Video 1:08:16

3258. Count Substrings That Satisfy K-Constraint I

Easy78.9% acceptance1 company

You are given a binary string s and an integer k.

A binary string satisfies the k-constraint if either of the following conditions holds:

The number of 0's in the string is at most k.
The number of 1's in the string is at most k.

Return an integer denoting the number of substrings of s that satisfy the k-constraint.

Example 1:

Input: s = "10101", k = 1

Output: 12

Explanation:

Every substring of s except the substrings "1010", "10101", and "0101" satisfies the k-constraint.

Example 2:

Input: s = "1010101", k = 2

Output: 25

Explanation:

Every substring of s except the substrings with a length greater than 5 satisfies the k-constraint.

Example 3:

Input: s = "11111", k = 1

Output: 15

Explanation:

All substrings of s satisfy the k-constraint.

Constraints:

1 <= s.length <= 50
1 <= k <= s.length
s[i] is either '0' or '1'.

Asked at 1 company

Google

Topics

Sliding Window String

Hints open one at a time

Hint 1

Using a brute force approach, check each index until a substring satisfying the k-constraint is found, then increment.

1:08:16

Still stuck?Read 3 approaches, or watch a walkthrough.

Open solution

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

Starter codePython 3Spaces: 4

Examples

Case 1Case 2Case 3

Input

"10101"
1

Input

"1010101"
2

Input

"11111"
1

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