What to Do in the First 5 Minutes of a Coding Interview

You lose most coding interviews in the first five minutes by writing code before you understand the input constraints. Stop nodding when the interviewer finishes the problem statement and immediately ask about array lengths, negative numbers, and duplicate entries.

When you hear a problem like Two Sum, your first instinct is to write a nested loop because it feels like progress. That instinct is a trap that leads to O(n^2) time complexity on the whiteboard. The interviewer wants to watch you peel back the layers of the prompt before a single character gets typed.

Ask the Three Constraints

You need to know the bounds of the input before you talk about algorithms. For an array problem, ask if nums can be empty, if the elements fit inside a standard integer, and if the array is already sorted. If the prompt says the array has up to 10^5 elements, an O(n^2) brute force approach fails immediately. If the array can contain negative values, a two-pointer approach from both ends might fail unless you manage the pointers carefully. Pin down these parameters out loud.

Trace a Worked Example

Do not use the example given in the problem statement. Invent your own input of size four or five to test the logic.

Take Merge Intervals. If your test input is [[1, 3], [2, 6], [8, 10], [15, 18]], walk through index 0 and index 1 by hand.

Compare 3 from the first interval with 2 from the second interval. Because 3 is greater than or equal to 2, you merge them into [1, 6] and store that in your working memory. Doing this on the scratchpad proves you understand that intervals must be sorted by their start times first, which shifts your mental model from an unsorted mess to a clean sweep.

Agree on the Function Signature

Write the function header and the return type on the screen before filling in the logic.

If you are solving Valid Palindrome, write bool isPalindrome(string s) and confirm the return type with the person across the table.

Ask if the input string can be passed by const reference to avoid unnecessary copies in C++, or if you should mutate it in place. This prevents a silent rewrite ten minutes later when the interviewer points out that your memory allocation violates the space complexity constraint.

The First Five Minutes Checklist

• Confirm if the input can be null, empty, or contain duplicate values.
• State the maximum expected input size to rule out brute force loops.
• Walk through a custom test case of length four on paper.
• Write the exact function signature and confirm the return type.
• State your intended time complexity target before writing any logic.

Practicing this discipline on DSA Tracker helps you build the habit of slowing down before you code. Pick any medium problem today and spend three full minutes on constraints before you touch the keyboard.

Keep going with practice problems and the online compiler.

Further reading: Hash tables on Wikipedia.