How to Fix Longest Palindromic Substring Errors

In this tutorial, you'll learn about How to Fix Longest Palindromic Substring Errors. We cover key concepts, practical examples, and best practices.

Fix longest palindromic substring errors when expand center approach has O(n^3) or odd/even handling wrong.

Quick Fix

Wrong

def lps(s):
    n=len(s); mx=''
    for i in range(n):
        for j in range(i,n):
            sub=s[i:j+1]
            if sub==sub[::-1] and len(sub)>len(mx): mx=sub
    return mx

O(n^3). Checking each substring for palindrome is expensive.

Right

def lps(s):
    n=len(s); start=0; ml=1
    def expand(l,r):
        nonlocal start,ml
        while l>=0 and r<n and s[l]==s[r]:
            if r-l+1>ml: start=l; ml=r-l+1
            l-=1; r+=1
    for i in range(n):
        expand(i,i)   # odd length
        expand(i,i+1) # even length
    return s[start:start+ml]

s='babad' -> 'bab' or 'aba'. s='cbbd' -> 'bb'. O(n^2).

Prevention

Expand around center for odd (i,i) and even (i,i+1) length palindromes. O(n^2).

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