Free AI LeetCode Communication Coach — Longest Substring Without Repeating Characters

The problem

Given a string s, find the length of the longest substring without repeating characters.

Example: s = "abcabcbb" → 3 (abc)

Name left, right, and the last-seen map.

last = {}
left = 0
best = 0
for right, ch in enumerate(s):
    if ch in last and last[ch] >= left:
        left = last[ch] + 1
    last[ch] = right
    best = max(best, right - left + 1)
return best

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 grow a window until a character repeats.”
• “Then I move the left edge forward.”

Practise out loud

1. 1. Restate

   I need the length of the longest stretch of characters with no repeats.

2. 2. Approach

   I will use a sliding window and a map of the last index of each character. When I see a repeat inside the window, I move left past it.

3. 3. Example

   On abcabcbb, the window grows to abc length 3, then repeats force left forward. The max stays 3.

4. 4. Time and space

   Time is O(n). Space is O(alphabet size) for the map.

5. 5. Edge cases

   I would check empty input and a string of all the same letter.

Common mistakes when explaining Longest Substring Without Repeating Characters

• Saying “sliding window” without moving left on a duplicate.
• Returning the substring when the question asks for length.