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Longest Substring Without Repeating Characters at Apple — Interview Guide

How to approach Longest Substring Without Repeating Characters in a Apple interview: the optimal approach, what Apple interviewers look for, and company-specific tips.

Updated 2026-09MediumSliding Window

The problem at Apple

Given a string, find the length of the longest substring that contains no repeating characters.

Apple uses this problem to assess craft, detail, and domain depth. Expect Apple-specific follow-ups that tie the solution back to production reasoning and the company's engineering bar.

Key concepts

Maintain a window [left, right] that always holds unique characters.
When a duplicate is found, advance left until the window is valid again.
Track the maximum window length seen.

Approach

Use a hash map from character to its most recent index.
Expand the right pointer; if the character was seen at or after left, jump left past it.
Update the answer with right - left + 1 each step.

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

Complexity

Metric	Value
Time	O(n)
Space	O(min(n, alphabet))

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