8 Common Mistakes in Coding Interviews (And How to Avoid Them)
Many candidates fail coding interviews not because they lack skill, but because they make avoidable mistakes. These eight mistakes show up repeatedly in post-interview feedback. Learn them, and you will stand out by doing the opposite.
1. Jumping to Code Too Fast
The moment you hear the problem, you start typing. This leads to wrong assumptions, backtracking, and messy code. Interviewers want to see structured thinking first.
How to avoid:
Restate the problem, ask 1–2 clarifying questions, outline your approach, and walk through a small example. Only then start coding. See our How to Explain Your Approach guide.
2. Not Asking Clarifying Questions
"Can the array be empty?" "Are there duplicates?" "What is the range of values?" Skipping these questions leads to solutions that fail edge cases or solve the wrong problem.
How to avoid:
Always ask about: input size and range, edge cases (empty, single element), expected output when no solution exists, and whether the input can be modified.
3. Choosing the Wrong Pattern
You use Sliding Window when Prefix Sum is needed, or BFS when DFS is simpler. Wrong pattern means wrong complexity, wrong solution, or unnecessary struggle.
How to avoid:
Use our Decision Tree to practice pattern identification. Key distinction: contiguous subarray with sum K and negatives → Prefix Sum; contiguous subarray with at most K distinct → Sliding Window.
4. Off-by-One Errors
Loop bounds, array indices, and inclusive vs. exclusive ranges cause more bugs than algorithm logic. i < n vs i <= n matters.
How to avoid:
Test with n=1 and n=2. Use consistent conventions: 0-indexed arrays, half-open ranges [left, right). Before submitting, trace the first and last iteration mentally.
5. Ignoring Space Complexity
You get O(n) time but use O(n²) space with a naive DP. Or you use extra space when O(1) is possible. Interviewers often ask for optimizations.
How to avoid:
State both time and space complexity upfront. Know when 2D DP can be reduced to 1D, when recursion can be tail-optimized, and when you can reuse input space.
6. Not Testing Your Code
You finish coding and say "done" without running a single test case. The interviewer finds bugs you could have caught yourself.
How to avoid:
Always run 1–2 examples: a normal case and an edge case (empty, single element). Walk through the code step by step. Fix any bugs before declaring you are done.
7. Giving Up Too Early
You hit a roadblock and say "I do not know." Interviewers expect you to struggle a bit. They want to see how you handle difficulty.
How to avoid:
Say what you have tried. Ask for a hint. Start with a brute-force solution and optimize. Partial progress is better than silence.
8. Poor Variable Naming and Structure
Single-letter variables, no structure, and magic numbers make your code hard to follow. Interviewers read your code to understand your thinking.
How to avoid:
Use descriptive names: left, right for pointers; maxSum, windowSize for state. Extract complex logic into helper functions. Add a brief comment for non-obvious steps.
Quick Checklist Before Every Interview
□ Restate the problem and ask clarifying questions
□ Outline approach and complexity before coding
□ Walk through a small example
□ Use the correct pattern (check decision tree)
□ Test with normal and edge cases
□ Use clear variable names
Related reads:
Practice pattern recognition
Avoid mistake #3 by training with our quiz and decision tree.