Free AI LeetCode Communication Coach — Two Sum

The problem

Given an array of integers nums and an integer target, return the indices of the two numbers that add up to target. Exactly one solution. You may not use the same element twice.

Example: nums = [2,7,11,15], target = 9 → [0,1]

Narrate brute force, then a hash map of values already seen.

seen = {}
for i, n in enumerate(nums):
    need = target - n
    if need in seen:
        return [seen[need], i]
    seen[n] = i

How to explain it

1. 1. Restate

   Say the problem in your own words.

   One or two sentences. Show you understood the input, the output, and the goal — not that you memorised the prompt.

2. 2. Approach

   Name the method before you code.

   Brute force first if you need it, then the structure you will use: hash map, two pointers, stack, binary search.

3. 3. Example

   Walk one concrete input.

   Pick small numbers. Say what you store, what you compare, and what you return. Interviewers follow an example more easily than abstract talk.

4. 4. Time and space

   One sentence each.

   After the example, before you claim you are done. “Time is O(n) because we scan once. Space is O(n) for the map.”

5. 5. Edge cases

   Name at least one unusual input.

   Empty input, duplicates, already sorted, overflow. Invite a follow-up: “I would also check …”

• “Let me restate this.”
• “My first instinct is brute force, then I will optimise.”
• “I will keep a hash map of values I have already seen.”
• “Time is O(n). Space is O(n) for the map.”

Practise out loud

1. 1. Restate

   Let me restate this. I have an array and a target. I need the two indices that add up to the target, and I cannot reuse the same index.

2. 2. Approach

   Brute force is check every pair — correct but O(n²). I will scan once and keep a hash map of values I have already seen.

3. 3. Example

   For example, nums are 2, 7, 11, 15 and target is 9. I see 2, store index 0. At 7 I need 2, the map has it, so I return 0 and 1.

4. 4. Time and space

   Time is O(n) because I scan once. Space is O(n) for the map.

5. 5. Edge cases

   I would also check the empty array and the case where the pair sits at the two ends.

Common mistakes when explaining Two Sum

• Jumping to code without restating the pair-of-indices goal.
• Saying “hash map” without walking a concrete example.
• Forgetting space is O(n) for the map.