mediumDSA

How do you find the longest substring without repeating characters?

Q: How do you find the longest substring without repeating characters?

**Intuition.** Slide a **window** across the string. Expand the right edge to include new characters; the instant you hit a repeat, jump the left edge to just past the previous occurrence of that character. Track the widest window seen. Each character is visited at most twice → O(n). "abcabcbb" → longest unique window "abc" (len 3) a <rect x="92" y="44" width="

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Explain the sliding-window approach.

stringssliding-window

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Sliding window with last-seen map

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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

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