Find longest substring without repeating characters

Question

Given a string S of length N find longest substring without repeating characters.

Example:

Input:

Answer 1

can we use something like this .

def longestpalindrome(str1):
    arr1=list(str1)
    s=set(arr1)
    arr2=list(s)
    return len(arr2)



str1='abadef'
a=longestpalindrome(str1)
print(a)

if only length of the substring is to be returned