Is Select K Disjoint Special Substrings easy or hard?

The problem is rated Medium but has a relatively low acceptance rate because identifying valid substring intervals is tricky. Once intervals are constructed correctly, the rest reduces to a standard greedy interval scheduling problem.

Select K Disjoint Special Substrings Python/Java solution

Typical implementations compute character boundaries with an array or map, generate valid intervals by expanding ranges, then sort intervals by end index. A greedy loop selects the maximum number of non-overlapping substrings and checks whether the count is at least k. The same logic works consistently in Python, Java, C++, and Go.

How to solve Select K Disjoint Special Substrings in O(n)?

Compute the first and last occurrence of each character and expand intervals only from first occurrences. Each character is processed a constant number of times while expanding intervals. If interval sorting is avoided using bucket ordering or alphabet constraints, the approach can approach O(n) time in practice, though most implementations remain O(n log n).

What is the best approach for Select K Disjoint Special Substrings?

The most effective solution builds minimal valid character intervals and then selects non-overlapping ones using a greedy interval scheduling strategy. First compute first and last occurrences of every character, expand intervals to include all occurrences of characters inside them, then sort by end index and greedily pick intervals. This runs in O(n log n) time and O(n) space.

Is Select K Disjoint Special Substrings asked at Google/Amazon/Meta?

Interval scheduling combined with character boundary preprocessing appears frequently in interviews at large tech companies. Variants of this pattern show up at companies like Google and Amazon because they test greedy reasoning, string preprocessing, and interval reasoning in one problem.

What data structure is used in Select K Disjoint Special Substrings?

The solution mainly uses hash tables or arrays to store the first and last occurrence of characters. After generating candidate intervals, sorting and greedy selection are applied. Some implementations also use dynamic programming with binary search for interval compatibility.

What is the time complexity of Select K Disjoint Special Substrings?

The optimal solution runs in O(n log n) time due to sorting candidate intervals. Building character boundaries and expanding intervals takes O(n). Space complexity is O(n) for storing interval candidates and character occurrence data.

Description Hints 3 Video 29:21

3458. Select K Disjoint Special Substrings

Medium19.2% acceptance1 company

Given a string s of length n and an integer k, determine whether it is possible to select k disjoint special substrings.

A special substring is a substring where:

Any character present inside the substring should not appear outside it in the string.
The substring is not the entire string s.

Note that all k substrings must be disjoint, meaning they cannot overlap.

Return true if it is possible to select k such disjoint special substrings; otherwise, return false.

Example 1:

Input: s = "abcdbaefab", k = 2

Output: true

Explanation:

We can select two disjoint special substrings: "cd" and "ef".
"cd" contains the characters 'c' and 'd', which do not appear elsewhere in s.
"ef" contains the characters 'e' and 'f', which do not appear elsewhere in s.

Example 2:

Input: s = "cdefdc", k = 3

Output: false

Explanation:

There can be at most 2 disjoint special substrings: "e" and "f". Since k = 3, the output is false.

Example 3:

Input: s = "abeabe", k = 0

Output: true

Constraints:

2 <= n == s.length <= 5 * 10⁴
0 <= k <= 26
s consists only of lowercase English letters.

Asked at 1 company

Amazon

Topics

Hints open one at a time

Hint 1

There are at most 26 start points (which are the first occurrence of each letter) and at most 26 end points (which are the last occurrence of each letter) of the substring.

Hint 2

Starting from each character, build the smallest special substring interval containing it.

Hint 3

Use dynamic programming on the obtained intervals to check if it's possible to pick at least k disjoint intervals.