10 Common Coding Interview Mistakes That Cost You the Job

The Most Common Interview Mistakes

After analyzing thousands of coding interviews, we've identified the most common mistakes that cost candidates their dream jobs. Here's how to avoid them:

1. Not Asking Clarifying Questions

Many candidates jump straight into coding without understanding the problem requirements. Always ask about edge cases, input constraints, and expected output format.

Good Example:

"Should I handle negative numbers? What if the array is empty? Should I return null or an empty array?"

2. Starting to Code Immediately

Interviewers want to see your thought process. Take time to explain your approach, discuss different solutions, and walk through examples before coding.

Good Approach:

"I'll use a two-pointer approach. Let me walk through an example with [1,2,3,4,5] to show how it works..."

3. Not Testing Your Code

Always test your solution with the provided examples and edge cases. Walk through your code step by step to catch bugs before the interviewer does.

Testing Process:

"Let me trace through this with input [2,7,11,15] and target 9. At i=0, j=1, we have 2+7=9, so we return [0,1]."

4. Ignoring Edge Cases

Edge cases often reveal bugs in your logic. Always consider empty inputs, single elements, duplicates, and boundary conditions.

• Empty array or null input
• Single element arrays
• Duplicate values
• Maximum/minimum values
• Negative numbers

5. Not Optimizing Your Solution

Start with a brute force solution, then optimize. Interviewers want to see your problem-solving progression and optimization skills.

Optimization Process:

"My brute force solution is O(n²). I can optimize this to O(n) using a hash map to store complements."

6. Poor Variable Naming

Use descriptive variable names that make your code self-documenting. Avoid single-letter variables unless they're standard (i, j for loops).

Bad:

for (int i = 0; i < n; i++) { int x = arr[i]; }

Good:

for (int index = 0; index < nums.length; index++) { int currentNumber = nums[index]; }

7. Not Explaining Your Thought Process

Communication is key. Explain what you're thinking, why you chose a particular approach, and what trade-offs you're considering.

8. Getting Stuck and Giving Up

If you get stuck, don't panic. Ask for hints, try a different approach, or start with a simpler version of the problem.

9. Not Considering Time and Space Complexity

Always analyze and state the time and space complexity of your solution. This shows you understand algorithmic efficiency.

10. Not Handling Errors Gracefully

Consider what happens when your function receives invalid input. Add appropriate error handling and validation.

How to Avoid These Mistakes

Practice Makes Perfect

✓Practice explaining your approach out loud
✓Always test your solutions with examples
✓Start with brute force, then optimize
✓Ask clarifying questions before coding
✓Use descriptive variable names