What to Do When You Don't Understand a LeetCode Problem

You open a LeetCode problem. You read it once. Twice. Three times.

"Wait, what does this even mean?"

The problem uses weird terminology. The constraints are buried in a wall of text. The examples don't help. You feel stuck before you even write a single line of code.

This is not a sign that you're bad at coding. It's a sign that you need a comprehension framework.

Here's the exact process to decode any confusing problem.

TL;DR

• Don't read passively. Actively annotate: underline inputs, outputs, constraints, and edge cases.
• Restate the problem in your own words. If you can't, you don't understand it yet.
• Trace the examples step-by-step by hand. This often reveals the hidden logic.
• Ask clarifying questions (even to yourself) about ambiguous parts: "Can the input be empty?" "Are numbers sorted?"
• Break it down into the smallest possible sub-task. Start with "What would I do for the first element?"

Step 1: Slow Down and Annotate

Your first instinct is to skim. Resist it.

What to Do

Read the problem sentence by sentence and highlight:

1. Input format: What am I given? (An array? A tree? A string?)
2. Output format: What should I return? (An integer? A boolean? A new array?)
3. Constraints: What are the limits? (Size? Value ranges? Time complexity?)
4. Edge cases: What happens if input is empty, has duplicates, or is at max size?

Example: "Find the Longest Substring Without Repeating Characters"

Annotated:

• Input: A string s.
• Output: An integer (length of longest substring).
• Constraint: Substring means contiguous (this is critical!).
• Edge case: What if s is empty? (Return 0.)

Just doing this forces you to engage with the problem actively instead of hoping it magically makes sense.

Step 2: Restate It in Your Own Words

If you can't explain the problem to a friend (or a rubber duck), you don't understand it.

The Test

Close the problem tab and say out loud (or write down):

"I need to [do what] given [input] and return [output]."

Example:

"I need to find the length of the longest substring where all characters are unique, given a string."

If you stumble on this, go back and re-read until you can say it smoothly.

Step 3: Manually Trace the Examples

Most LeetCode problems give you 1-3 examples. These are gold. They show you how the problem works.

How to Trace

Pick the first example. Pretend you are the algorithm.

1. Write down the input.
2. Step through it, one element at a time.
3. Track intermediate states (variables, data structures).
4. Confirm your final answer matches the expected output.

Example: Two Sum

• Input: nums = [2, 7, 11, 15], target = 9
• Step-by-step:
  • Look at 2. Need 7 to sum to 9. Is 7 in the rest? Yes, at index 1.
  • Return [0, 1].

What you learned: You're checking if target - current exists. That's a hash map problem!

Step 4: Ask Clarifying Questions (Yes, to Yourself)

In a real interview, you'd ask the interviewer. When practicing alone, ask yourself these questions:

Write down your assumptions. If you were in an interview, this is where you'd get clarification. When practicing, make reasonable assumptions and note them in comments.

Step 5: Break It Down to the Smallest Piece

If the problem still feels overwhelming, shrink it.

The Approach

Instead of solving the entire problem, solve a tiny version first.

Example: "Merge K Sorted Lists"

• Full problem: Merge K sorted linked lists.
• Tiny version: Can you merge 2 sorted linked lists? (Yes? Good, that's 80% of the solution.)

Example: "Minimum Window Substring"

• Full problem: Find the smallest substring containing all characters from t.
• Tiny version: Can you check if a substring contains all characters? (Yes? Now add sliding window logic.)

Once you solve the simpler version, the full problem becomes "repeat this logic with a smarter iteration."

Common Mistakes When Reading Problems

Mistake 1: Skipping Constraints

You code a solution that works for small inputs but times out on large ones because you ignored "1 ≤ n ≤ 10^5."

Fix: Always check constraints. If n ≤ 10^5, you can't do O(n²) brute force.

Mistake 2: Misunderstanding "Subarray" vs "Subsequence"

• Subarray: Contiguous elements (e.g., [2, 3] from [1, 2, 3, 4]).
• Subsequence: Non-contiguous but order-preserved (e.g., [1, 3, 4] from [1, 2, 3, 4]).

Mixing these up changes the entire problem.

Mistake 3: Assuming Inputs Are Always Valid

LeetCode usually states if inputs are valid, but edge cases like empty arrays, null pointers, or negative numbers can trip you up.

Fix: Write a quick "sanity check" block at the start of your code.

How Tools Can Help

After you've tried to understand the problem yourself, it's okay to get help.

For example, LeetCopilot's Chat Mode lets you paste the problem statement and ask:

"Can you rephrase this problem in simpler terms?"

Or:

"What are the edge cases I should consider?"

It's like having a study partner who won't judge you for asking "dumb" questions.

FAQ

Q: How long should I spend trying to understand a problem before looking at hints?
A: 10-15 minutes. If you truly can't parse the problem statement after that, it's okay to seek help.

Q: What if the problem uses unfamiliar terms (like "trie" or "heap")?
A: That's a signal to pause and research the data structure first. You can't solve a problem if you don't know the building blocks.

Q: Should I always understand the problem 100% before coding?
A: Aim for 90%. Sometimes you clarify the last 10% by trying to code and hitting a wall. That's fine.

Q: What if I understand the problem but have no idea how to solve it?
A: That's a different issue (pattern recognition). But at least you've cleared the first hurdle!

Conclusion

You can't solve a problem you don't understand.

But "understanding" isn't passive. It's active deconstruction:

1. Annotate the problem.
2. Restate it in your own words.
3. Trace the examples by hand.
4. Ask clarifying questions.
5. Break it into the smallest solvable piece.

Master this framework, and you'll never feel paralyzed by a confusing problem statement again.