How to Fix Longest Substring Without Repeating Errors

In this tutorial, you'll learn about How to Fix Longest Substring Without Repeating Errors. We cover key concepts, practical examples, and best practices.

Fix longest substring without repeating errors when character position reset wrong or left pointer doesn't jump to after duplicate.

Quick Fix

Wrong

def lnrc(s):
    l=mx=0; seen=set()
    for r in range(len(s)):
        while s[r] in seen:
            seen.remove(s[l]); l+=1
        seen.add(s[r]); mx=max(mx,r-l+1)
    return mx

While loop removes characters one by one. O(n) amortized but can be O(n^2) worst case.

Right

def lnrc(s):
    last={}; l=0; mx=0
    for r,c in enumerate(s):
        if c in last and last[c]>=l:
            l=last[c]+1  # jump left past duplicate
        last[c]=r; mx=max(mx,r-l+1)
    return mx

'abcabcbb' -> 3 ('abc'). 'bbbbb' -> 1 ('b'). 'pwwkew' -> 3 ('wke'). O(n).

Prevention

Track last seen index of each character. Jump left pointer to last occurrence+1 on duplicate.

DodaTech Tools

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