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mediumDSA

How do you find the longest substring without repeating characters?

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01

Understand the problem

Explain the sliding-window approach.

stringssliding-window
02

Attempt it yourself

Sketch your approach before reading the solution — that's what interviews test.

Nudge consolestandby

Stuck? Beam a request up — the console returns a conceptual nudge that guides your logic without spoiling the implementation.

03

Study the solution

Expand a sliding window with a set/map of last-seen indices. When you hit a repeat, jump the left edge past the previous occurrence. Track the max window length throughout — O(n) time, O(min(n, charset)) space.

Solution ready — 2 min read

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04

Read the code

Sliding window with last-seen map
Run Playground
def length_of_longest(s):
    last = {}
    left = 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


# --- demo ---
print(length_of_longest("abcabcbb"))   # 3  ("abc")
print(length_of_longest("bbbbb"))      # 1
05

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