Free AI LeetCode Communication Coach — Valid Palindrome

The problem

Given a string s, return true if it is a palindrome after converting to lowercase and removing non-alphanumeric characters.

Example: s = "A man, a plan, a canal: Panama" → true

Say skip junk, then compare.

left, right = 0, len(s) - 1
while left < right:
    while left < right and not s[left].isalnum():
        left += 1
    while left < right and not s[right].isalnum():
        right -= 1
    if s[left].lower() != s[right].lower():
        return False
    left += 1
    right -= 1
return True

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 will use two pointers from both ends.”
• “I skip non-alphanumeric characters.”

Practise out loud

1. 1. Restate

   I need to check if the string reads the same forwards and backwards, ignoring case and non-letters.

2. 2. Approach

   I will use two pointers from both ends. I skip anything that is not alphanumeric, then compare lowercase characters.

3. 3. Example

   On “A man, a plan, a canal: Panama”, left starts at A, right at a. After skips they match through the middle, so it is valid.

4. 4. Time and space

   Time is O(n). Space is O(1) because I do not build a new string.

5. 5. Edge cases

   I would check an empty string and a string that is only punctuation.

Common mistakes when explaining Valid Palindrome

• Forgetting to skip non-alphanumeric characters out loud.
• Saying reverse without mentioning extra space.