How to explain code in interview?

Explaining your code effectively during an interview is just as important as writing it. It demonstrates your ability to communicate technical concepts clearly, shows your thought process, and helps the interviewer understand how you approach problem-solving. Here’s how to explain your code effectively during an interview:

1. Start by Explaining Your Approach

Before diving into writing code, outline your approach to solving the problem. This gives the interviewer a roadmap of your thought process and lets them follow along with your logic.

Steps:

• Restate the Problem Briefly: Ensure both you and the interviewer are on the same page. You can say something like:
  • "So, the goal is to find the longest substring without repeating characters, right?"
• Describe Your Plan: Explain how you intend to solve the problem step by step before you write any code. Mention any patterns, algorithms, or techniques you plan to use.
  • "I’ll use a sliding window approach where I maintain two pointers to track the start and end of the current substring."

This shows that you understand the problem and can articulate a high-level plan before diving into the details.

2. Break the Code into Logical Steps

As you write the code, break it down into clear, logical steps and explain each part as you go. This helps the interviewer follow your thought process and understand why you’re doing what you’re doing.

Example:

For a problem like "Find the longest substring without repeating characters," explain each step as you code:

• Initializing Variables:
  • "I’ll start by initializing two pointers, left and right, both at the start of the string, and a hash map to track characters."
• Looping Through the String:
  • "Then, I’ll loop through the string with the right pointer, adding characters to the hash map and checking for repeats."
• Sliding the Window:
  • "If I encounter a repeat, I’ll adjust the left pointer to shrink the window, ensuring no repeats."

3. Explain Your Thought Process

As you write the code, explain why you’re making specific decisions. This shows your reasoning behind each step and helps the interviewer understand how you approach problem-solving.

Examples:

• Why Use a Particular Data Structure:
  • "I’m using a hash map here because it allows constant time lookups to check if a character has already been seen."
• Edge Case Consideration:
  • "I’ll handle the case where the input string is empty by returning 0 right at the start."
• Optimization Decisions:
  • "By using a sliding window, I can reduce the time complexity to O(n) instead of O(n²), which would happen with a brute-force approach."

4. Use Clear and Simple Language

Avoid overly technical jargon that might make your explanation confusing. Instead, explain your code in simple terms that show your understanding of the concepts involved.

Example:

Instead of saying:

• "I’m using a logarithmic time complexity solution with amortized constant time operations on the data structure."

You could say:

• "I’m using this approach because it allows me to check and update the character positions quickly, which makes the solution faster for large inputs."

Simple, clear language helps the interviewer follow along easily, especially when you’re explaining complex ideas.

5. Mention Time and Space Complexity

After you’ve written and explained your code, analyze its efficiency by discussing the time and space complexity. Interviewers expect you to think about the performance of your solution, so mentioning complexity demonstrates that you’re mindful of it.

Steps:

• Time Complexity: Discuss how the code scales with the input size (e.g., O(n), O(log n), etc.).
  • "Since I’m iterating over the string only once, this solution runs in O(n) time, where n is the length of the string."
• Space Complexity: Mention how much extra memory your solution uses.
  • "The space complexity is O(min(n, m)), where m is the size of the character set, since we’re using a hash map to store character positions."

6. Acknowledge Edge Cases

Make sure to explain how your code handles edge cases, such as empty inputs, extremely large inputs, or invalid data. Interviewers like to see that you’ve thought about the robustness of your solution.

Example:

For the longest substring without repeating characters problem, mention:

• "If the string is empty, my code returns 0 immediately, since there’s no valid substring."
• "For strings with all unique characters, like 'abcdef', the window will expand to the full length."

7. Test Your Code with Example Inputs

After writing the code, walk through some test cases by explaining how the input would flow through the code and how the output is generated.

Steps:

• Pick Simple Test Cases: Use example inputs provided by the interviewer or create your own.
  • "Let’s take the string 'abcabcbb' as an example. The longest substring without repeating characters is 'abc', so my code should return 3."
• Explain What Happens at Each Step: Go through each step of your code with the example input, showing how the variables and logic change as the input is processed.
  • "First, the hash map is empty. Then, as I iterate over the string, I add 'a', 'b', and 'c' to the map. When I encounter a second 'a', I slide the window by moving the left pointer."

8. Be Open to Feedback and Adjustments

Sometimes, the interviewer may suggest improvements or alternate approaches to the problem. Be open to feedback, and if you can improve your solution based on their suggestions, explain how and why you would make changes.

Example:

• "That’s a good point. I could optimize this further by using a set instead of a hash map since I’m only concerned with whether the character exists."

This shows that you can think critically about your code and are willing to adapt when necessary.

9. Stay Calm and Confident

Even if you’re not 100% sure about your solution or make a mistake while explaining, remain calm. Take your time, and if you get stuck, verbalize your thought process. The interviewer values how you think through problems just as much as your final code.

Example:

• "I realize there might be a more efficient way to handle this case. Let me take a moment to think through an optimization."

10. Summarize Your Solution

Once you’ve finished coding and testing, provide a brief summary of your solution and its key features.

Example:

• "To summarize, I used a sliding window approach with two pointers and a hash map to track character positions. This ensures that I can find the longest substring in linear time while keeping space complexity minimal."

Conclusion

Explaining code in an interview is about more than just writing correct code—it’s about showing your problem-solving process, reasoning, and communication skills. By breaking the problem into steps, explaining your choices, discussing edge cases, and analyzing the code’s complexity, you demonstrate not only your technical abilities but also your ability to work through challenges in a structured way. Always remember that communication is key in coding interviews, so practice explaining your solutions clearly and confidently.