Coding problem: Pair With Target Sum. You are given a list of integers `nums` and an integer `target`. Return the positions of the two different entries that add up to `target`, as a list `[i, j]` with `i < j`.

What the interviewer is looking for

• What a strong solution looks like: one pass with a hash map from value to index; for each x, check whether target - x was already seen, then store x. O(n) time, O(n) space. Sorting with two pointers is O(n log n) and needs care to keep the original positions.
• Think aloud. Interviewers are judging how you reason, not only the final code.
• Be ready for: What if there may be no pair, or many — return all distinct value pairs? If the list is sorted, can you do it in O(1) extra space? What if the numbers don't fit in memory?

A structure to follow: Clarify, plan, code, test, analyse

1. Step 1

   Clarify

   Restate the problem, ask about empty input, duplicates and size limits, and agree on one example.

2. Step 2

   Plan

   Say the simple approach first and its cost, then the better one. Explain why before you type.

3. Step 3

   Code

   Write small named steps and talk through each. A clear solution beats a clever one.

4. Step 4

   Test

   Walk your own example through the code, then an edge case. Fix what you find calmly.

5. Step 5

   Analyse

   State time and space cost in plain words and say what you would change for much larger input.

This is guidance on shape, not a script. Use your own real experience and facts; never invent details to fill a gap. See the STAR guide and answer frameworks.

Common slips

• Starting to code before agreeing what the answer must be.
• Going silent when stuck instead of saying what you are trying.
• Never running your own example through the code.

How to practise it

Solve it once in the editor without hints, then explain your solution aloud in under two minutes.

Related questions

• Coding problem: Balanced Brackets. Given a string `text`, decide whether its brackets are balanced. The bracket pairs are `()`, `[]` and `{}`; every other character is ignored.
• Coding problem: First Unrepeated Character. Given a string `text`, return the index of the first character that occurs exactly once in the whole string. Return `-1` if every character repeats.
• Coding problem: Palindrome After Cleanup. Decide whether `text` reads the same forwards and backwards once you drop every character that isn't a letter or a digit and ignore letter case.
• Coding problem: Totals by Category. Each record is an object `{"category": string, "amount": integer or null}` describing one sale, with the amount in cents. Return an object that maps each category to the sum of its amounts.
• Coding problem: Flatten Nested Settings. Given a nested object `obj` (for example, a settings tree), return a flat object whose keys are the dot-separated paths to every leaf value.

Browse all questions