Longest Substring Without Repeating Characters

Given a string s, your task is to find the longest substring of s that contains no repeating characters. If there are multiple substrings with the maximum length, the substring that appears first in s should be returned.

The problem can be mathematically described as follows: given a string \( s \), find indices \( i \) and \( j \) (with \( 0 \leq i \leq j < |s| \)) such that the substring \( s[i \ldots j] \) has all unique characters and \( j - i + 1 \) is maximized. If there exists another substring with the same maximum length, choose the one with the smallest starting index.

Examples:

• For s = "abcabcbb", the answer is "abc".
• For s = "bbbbb", the answer is "b".
• For s = "pwwkew", the answer is "wke".

inputFormat

The input consists of a single string s provided via standard input (stdin). The string can contain letters, digits, and special characters.

Input Format:

s

outputFormat

Output the longest substring of s with all unique characters to standard output (stdout). If s is empty, output an empty string.

Output Format:

substring

## sample

abcabcbb

abc