Free AI LeetCode Communication Coach — Contains Duplicate

The problem

Return true if any value appears at least twice in the array.

Example: nums = [1,2,3,1] → true

Set membership is the whole story.

seen = set()
for n in nums:
    if n in seen:
        return True
    seen.add(n)
return False

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 …”

• “I store values I have already seen.”
• “If insert fails, it is a duplicate.”

Practise out loud

1. 1. Restate

   I need to know if any number appears more than once.

2. 2. Approach

   I scan once and store values in a hash set. If I see a value already in the set, I return true.

3. 3. Example

   On 1,2,3,1 — when I see the second 1 the set already has it, so true.

4. 4. Time and space

   Time is O(n). Space is O(n) for the set.

5. 5. Edge cases

   I would check an empty array and an all-unique array.

Common mistakes when explaining Contains Duplicate

• Only sorting without mentioning the space trade-off.
• Not walking a small example.