#1316

Hard

LeetCode Problem Workspace

Distinct Echo Substrings

Count the distinct non-empty substrings of a given string that can be formed as the concatenation of a string with itself.

string trie rolling hash hash function

Problem Statement

Given a string text, you are tasked with returning the number of distinct non-empty substrings that can be expressed as a concatenation of some string with itself (i.e., as a + a, where a is a substring).

For example, in the string abcabcabc, the distinct echo substrings are "abcabc", "bcabca", and "cabcab". This problem requires efficient handling of substring matching using string and Trie-based methods.

Examples

Example 1

Input: text = "abcabcabc"

Output: 3

The 3 substrings are "abcabc", "bcabca" and "cabcab".

Example 2

Input: text = "leetcodeleetcode"

Output: 2

The 2 substrings are "ee" and "leetcodeleetcode".

Constraints

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

Solution Approach

String Matching with Trie

You can use a Trie to store substrings and efficiently check if they can be written as a concatenation of a string with itself. By building substrings iteratively, the Trie allows quick matching and insertion.

Rolling Hash for Efficient Substring Comparison

A rolling hash technique can be used to compute hash values of substrings in constant time, making substring comparisons faster and allowing you to check for repeated substrings efficiently.

Iterating Over Substrings

To ensure all substrings are examined, iterate over all possible starting points in the string, generating and checking substrings. Using a hash set or Trie will allow you to track distinct substrings.

Complexity Analysis

Metric	Value
Time	Depends on the final approach
Space	Depends on the final approach

The time and space complexities depend on the approach chosen. For the Trie-based solution, it involves storing substrings, leading to time complexity near O(n^2) where n is the length of the string. The space complexity is also O(n^2) due to storage requirements for all substrings.

What Interviewers Usually Probe

The candidate should demonstrate a strong grasp of string manipulation techniques like Trie usage and rolling hashes.
Look for understanding of how to handle distinct substrings efficiently.
Pay attention to how they balance time complexity with space usage, especially in terms of optimizing substring checks.

Common Pitfalls or Variants

Common pitfalls

Failing to account for overlapping substrings that are distinct but share characters, which can lead to incorrect results.
Not considering edge cases like very short strings or strings with only one character repeated multiple times.
Overcomplicating the problem by not optimizing substring checking, leading to poor performance for larger inputs.

Follow-up variants

Consider handling strings with uppercase letters or non-alphabetical characters.
Expand to handle substrings that can be written as multiple concatenations of a string, not just two.
Extend the problem to find the longest echo substring instead of just counting them.

FAQ

What is a distinct echo substring?

A distinct echo substring is a non-empty substring that can be written as the concatenation of a string with itself, such as 'abcabc'.

What approach should I use to find distinct echo substrings?

You can use a combination of Trie data structures and rolling hashes for efficient substring comparison and checking.

How can I optimize the space complexity for this problem?

Using a rolling hash technique or compressing substring storage can help reduce space complexity while still allowing efficient checks for distinct substrings.

What is the time complexity of the Trie-based approach for this problem?

The time complexity of a Trie-based approach is O(n^2), where n is the length of the string, due to storing and checking all possible substrings.

How does GhostInterview help with this problem?

GhostInterview provides tailored approaches and algorithmic strategies to efficiently find distinct echo substrings using advanced data structures and hashing techniques.

terminal

Solution

Solution 1

#### Python3

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class Solution:
    def distinctEchoSubstrings(self, text: str) -> int:
        def get(l, r):
            return (h[r] - h[l - 1] * p[r - l + 1]) % mod

        n = len(text)
        base = 131
        mod = int(1e9) + 7
        h = [0] * (n + 10)
        p = [1] * (n + 10)
        for i, c in enumerate(text):
            t = ord(c) - ord('a') + 1
            h[i + 1] = (h[i] * base) % mod + t
            p[i + 1] = (p[i] * base) % mod
        vis = set()
        for i in range(n - 1):
            for j in range(i + 1, n, 2):
                k = (i + j) >> 1
                a = get(i + 1, k + 1)
                b = get(k + 2, j + 1)
                if a == b:
                    vis.add(a)
        return len(vis)

Continue Practicing

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