Google · Coding 2 of 3

Longest Substring Without Repeating Characters

Find the length of the longest substring with no repeated character.

MediumStringSliding windowHash mapBest: O(n) time · O(min(n, alphabet)) space

Practice question: an editorial practice prompt, not a claim that Google asks this exact question. Interviews vary by role, level, team, and location.

Problem

Problem statement

Given a string s, return the length of the longest substring — a contiguous run of characters — in which no character appears twice.

Examples

Example 1
Input
s = "abcabcbb"
Output
3

Why: "abc" is the longest run without a repeat.

Example 2
Input
s = "bbbbb"
Output
1

Why: Every longer run repeats "b".

Example 3
Input
s = "pwwkew"
Output
3

Why: "wke". Note "pwke" is not contiguous, so it does not count.

Constraints

  • 0 ≤ s.length ≤ 5 × 10⁴
  • s may contain letters, digits, symbols and spaces.
In the interview

How to answer it out loud

What it tests: Sliding-window reasoning, choosing what state to keep, and avoiding the off-by-one when the left edge moves.

  1. 1
    Clarify

    Clarify the character set, whether case matters, and what an empty string returns.

  2. 2
    Implement

    Grow a window to the right and store each character’s last index; when a repeat falls inside the window, jump the left edge past it.

  3. 3
    Verify

    Trace "abba", "", and "pwwkew"; explain O(n) time and why the left edge never moves backwards.

Approaches

From brute force to the best solution

ApproachIdeaTimeSpace
Check every substringFor each start, extend until a repeat appears.O(n²)O(min(n, alphabet))
Sliding window + last-seen indexBestWhen a character repeats inside the window, jump the left edge past its previous position.O(n)O(min(n, alphabet))
Key insight

The left edge only ever moves forward. Only jump it when the previous copy is inside the current window — otherwise "abba" returns 3 instead of 2.

Solution

Reference solution

Each version is self-contained and has been run against the examples and test cases on this page.

def length_of_longest_substring(s: str) -> int:    last_seen = {}                       # char -> most recent index    best = left = 0    for right, ch in enumerate(s):        if last_seen.get(ch, -1) >= left:  # repeat inside the current window            left = last_seen[ch] + 1       # jump past the earlier copy        last_seen[ch] = right        best = max(best, right - left + 1)    return best
Time
O(n)
Space
O(min(n, alphabet))
Approach
Sliding window + last-seen index
Test cases

Edge cases to run before you say “done”

InputExpectedWhy it matters
s = ""0Empty input.
s = "abba"2The left edge must never move backwards.
s = "a b"3Spaces are characters too.
Likely follow-ups

Answer the main question first, then take these on one at a time:

  • What if you must return the substring, not its length?
  • What if each character may appear at most twice?
  • How would you handle a stream of characters?
Avoid these mistakes

Check your recording or written solution against this list:

  • Moving the left edge backwards on a stale index
  • Confusing a substring with a subsequence
  • Rebuilding a set for every start index
Sources and methodology

This guide combines general interview-practice patterns with the public hiring material below. The practice prompt and coaching are PiriPiri AI editorial content, not official Google questions or answers.

Google Careers: how we hire ↗
Official sources reviewed 20 September 2026.