TL;DR: The longest common substring is the longest contiguous sequence shared by two strings. A reliable interview solution uses a two-dimensional dynamic-programming table: matching characters extend the diagonal value by one, while mismatches reset the cell to zero. Track the largest value and its ending position to reconstruct the substring.
Longest Common Substring: Interview Guide and Solutions
Learn the longest common substring problem, compare dynamic programming with brute force, trace an example, and explain complexity in a coding interview.
Try YesToTheOfferWhat should you know first?
The longest common substring is the longest contiguous sequence shared by two strings. A reliable interview solution uses a two-dimensional dynamic-programming table: matching characters extend the diagonal value by one, while mismatches reset the cell to zero. Track the largest value and its ending position to reconstruct the substring.
Start by clarifying that substring means contiguous characters; it is not the longest common subsequence. For strings ABABC and BABCA, the answer is BABC with length 4. The direct brute-force approach checks starting pairs and extends matches, which can become cubic. Dynamic programming uses O(mn) time and O(mn) space, or O(n) space with a rolling row.
Continue with the AI interview copilot guide, resume-grounded answer preparation, and interview review workflow.
Which questions should you practice?
- What is the longest common substring?
- How does the dynamic-programming recurrence work?
- What is the time and space complexity?
- How is substring different from subsequence?
- How can memory be reduced?
Start by clarifying that substring means contiguous characters; it is not the longest common subsequence. For strings ABABC and BABCA, the answer is BABC with length 4. The direct brute-force approach checks starting pairs and extends matches, which can become cubic. Dynamic programming uses O(mn) time and O(mn) space, or O(n) space with a rolling row.
How do you build a strong answer?
The longest common substring is the longest contiguous sequence shared by two strings. A reliable interview solution uses a two-dimensional dynamic-programming table: matching characters extend the diagonal value by one, while mismatches reset the cell to zero. Track the largest value and its ending position to reconstruct the substring. Start by clarifying that substring means contiguous characters; it is not the longest common subsequence. For strings ABABC and BABCA, the answer is BABC with length 4. The direct brute-force approach checks starting pairs and extends matches, which can become cubic. Dynamic programming uses O(mn) time and O(mn) space, or O(n) space with a rolling row.
| What should you know first? | Show | Avoid |
|---|---|---|
| 1 | Direct answer and scope | A long autobiography |
| 2 | A concrete decision or example | Unsupported claims |
| 3 | Assumptions and tradeoffs | A rushed conclusion |
| 4 | One specific improvement | Vague self-criticism |
What does a focused preparation plan look like?
- What is the longest common substring?
- How does the dynamic-programming recurrence work?
- What is the time and space complexity?
- How is substring different from subsequence?
- How can memory be reduced?
Start by clarifying that substring means contiguous characters; it is not the longest common subsequence. For strings ABABC and BABCA, the answer is BABC with length 4. The direct brute-force approach checks starting pairs and extends matches, which can become cubic. Dynamic programming uses O(mn) time and O(mn) space, or O(n) space with a rolling row. The longest common substring is the longest contiguous sequence shared by two strings. A reliable interview solution uses a two-dimensional dynamic-programming table: matching characters extend the diagonal value by one, while mismatches reset the cell to zero. Track the largest value and its ending position to reconstruct the substring.
Which mistakes should you avoid?
- A long autobiography
- Unsupported claims
- A rushed conclusion
- Vague self-criticism
Use AI to organize material you already understand, rehearse explanations, and review your performance. Follow the employer's and assessment provider's rules, and never invent experience or results.
How can AI support responsible practice?
YesToTheOffer can ground preparation and real-time answer structure in your resume, job description, and private notes. It also supports coding work and keeps a transcript for post-interview review.
Use AI to organize material you already understand, rehearse explanations, and review your performance. Follow the employer's and assessment provider's rules, and never invent experience or results.
Continue with the AI interview copilot guide, resume-grounded answer preparation, and interview review workflow.
Frequently asked questions
FAQ
What is the longest common substring?
The longest common substring is the longest contiguous sequence shared by two strings. A reliable interview solution uses a two-dimensional dynamic-programming table: matching characters extend the diagonal value by one, while mismatches reset the cell to zero. Track the largest value and its ending position to reconstruct the substring.
How does the dynamic-programming recurrence work?
Start by clarifying that substring means contiguous characters; it is not the longest common subsequence. For strings ABABC and BABCA, the answer is BABC with length 4. The direct brute-force approach checks starting pairs and extends matches, which can become cubic. Dynamic programming uses O(mn) time and O(mn) space, or O(n) space with a rolling row.
What is the time and space complexity?
Start by clarifying that substring means contiguous characters; it is not the longest common subsequence. For strings ABABC and BABCA, the answer is BABC with length 4. The direct brute-force approach checks starting pairs and extends matches, which can become cubic. Dynamic programming uses O(mn) time and O(mn) space, or O(n) space with a rolling row.
How is substring different from subsequence?
The longest common substring is the longest contiguous sequence shared by two strings. A reliable interview solution uses a two-dimensional dynamic-programming table: matching characters extend the diagonal value by one, while mismatches reset the cell to zero. Track the largest value and its ending position to reconstruct the substring.
How can memory be reduced?
Start by clarifying that substring means contiguous characters; it is not the longest common subsequence. For strings ABABC and BABCA, the answer is BABC with length 4. The direct brute-force approach checks starting pairs and extends matches, which can become cubic. Dynamic programming uses O(mn) time and O(mn) space, or O(n) space with a rolling row.
Practice with evidence from your experience
The longest common substring is the longest contiguous sequence shared by two strings. A reliable interview solution uses a two-dimensional dynamic-programming table: matching characters extend the diagonal value by one, while mismatches reset the cell to zero. Track the largest value and its ending position to reconstruct the substring. Start by clarifying that substring means contiguous characters; it is not the longest common subsequence. For strings ABABC and BABCA, the answer is BABC with length 4. The direct brute-force approach checks starting pairs and extends matches, which can become cubic. Dynamic programming uses O(mn) time and O(mn) space, or O(n) space with a rolling row.
Practice with evidence from your experience
YesToTheOffer can ground preparation and real-time answer structure in your resume, job description, and private notes. It also supports coding work and keeps a transcript for post-interview review.
Try YesToTheOffer