Is Distinct Echo Substrings easy or hard?

Distinct Echo Substrings is classified as a Hard problem on LeetCode. It requires knowledge of rolling hash or advanced dynamic programming for substring comparison, along with careful handling of duplicates using hashing or sets.

Distinct Echo Substrings Python/Java solution

Python and Java implementations typically use rolling hash with prefix arrays and a set to store unique substring hashes. The algorithm iterates over possible half-lengths and starting indices, compares hash values of the two halves, and counts distinct matches.

How to solve Distinct Echo Substrings in O(n)?

No widely accepted O(n) algorithm exists for this problem. Most competitive programming and interview solutions rely on rolling hash or longest common prefix dynamic programming, both running in O(n^2) time. These methods efficiently compare substring halves while avoiding repeated character comparisons.

What is the best approach for Distinct Echo Substrings?

The rolling hash approach is typically the best solution. It compares the two halves of candidate substrings using prefix hashes, allowing O(1) equality checks. Iterating over all half-lengths and start positions results in O(n^2) time with a hash set used to store distinct echo substrings.

Is Distinct Echo Substrings asked at Google/Amazon/Meta?

Problems involving substring hashing and repeated patterns appear frequently in interviews at companies like Google and Meta. Distinct Echo Substrings specifically tests rolling hash, substring comparison, and deduplication techniques commonly expected in advanced string problems.

What data structure is used in Distinct Echo Substrings?

Common solutions use hash sets to track unique substrings and prefix hash arrays for rolling hash calculations. Some implementations also use a dynamic programming table to compute longest common prefixes between index pairs.

What is the time complexity of Distinct Echo Substrings?

The optimal known solutions run in O(n^2) time. Both rolling hash and dynamic programming approaches check substring pairs while ensuring fast equality checks. Space complexity is usually O(n^2) due to storing substrings or maintaining a DP table.

Description Hints 3 Solution Video 27:57

1316. Distinct Echo Substrings

Hard53.7% acceptance1 company

Return the number of distinct non-empty substrings of text that can be written as the concatenation of some string with itself (i.e. it can be written as a + a where a is some string).

Example 1:

Input: text = "abcabcabc"
Output: 3
Explanation: The 3 substrings are "abcabc", "bcabca" and "cabcab".

Example 2:

Input: text = "leetcodeleetcode"
Output: 2
Explanation: The 2 substrings are "ee" and "leetcodeleetcode".

Constraints:

1 <= text.length <= 2000
text has only lowercase English letters.

Asked at 1 company

Google

Topics

Hints open one at a time

Hint 1

Given a substring of the text, how to check if it can be written as the concatenation of a string with itself ?

Hint 2

We can do that in linear time, a faster way is to use hashing.

Hint 3

Try all substrings and use hashing to check them.

27:57

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