Longest Substring Without Repeating Characters

Problem

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

Examples

The brute-force approach

Try every possible starting index. From each start, extend as far right as possible while characters are unique. Track the maximum length found.

max_len = 0

for i in 0 .. n - 1:
    seen = set()
    for j in i .. n - 1:          # ← nested loop
        if s[j] in seen:
            break
        seen.add(s[j])
        max_len = max(max_len, j - i + 1)

return max_len

O(n²), for each start position, you rebuild a set and scan forward. The sliding window approach reuses work across iterations instead of starting from scratch each time.

Spotting the pattern

This is a Sliding Window problem. The key question to ask yourself:

Can I maintain a window of valid characters and slide it across the string in one pass?

Answering that is where it clicks, and it's exactly what the guided walkthrough below builds with you: the pattern reasoning, a progressive hint ladder that never spoils the answer, a row-by-row dry run, the optimized solution, and an in-browser editor to run your code against real test cases.