Longest Unique Substring

Given a string s, your task is to find the longest substring that contains no duplicate characters. Use a sliding window approach to ensure an efficient solution. Formally, for a string \( s \), find a substring \( s[i \ldots j] \) such that all characters are unique and \( j-i+1 \) is maximized. If there are multiple answers, return the first occurrence.

For example:

• For s = "abcabcbb", the longest substring is "abc" with length 3.
• For s = "pwwkew", the longest substring is "wke".

The problem can be formally defined using the following equation:

\[ \text{maximize} \quad L = j - i + 1 \quad \text{subject to } \forall k, l \in [i,j],\; s[k] \neq s[l] \text{ for } k \neq l. \]

inputFormat

The input consists of a single line containing the string s. The string s may include any printable characters.

outputFormat

Output the longest substring of s that does not contain any repeating characters. If multiple answers exist, output the one that appears first.

abcabcbb

abc