Shortest Substring with All Characters

Determine the shortest substring of the string S that includes every character from S at least once. If multiple shortest substrings exist, return the one that starts the earliest in S.

Input:

The input contains a single string 'S' consisting of only lowercase English letters.

Output:

Output the shortest substring of 'S' that contains all unique characters from 'S'.

Example:

Input:

"abcba"

Output:

"abc"

Explanation:

In the string S = "abcba", the substrings "abc" and "cba" both contain all characters of S. "abc" begins at index 0, earlier than "cba" which begins at index 2. Hence, the answer is "abc".

Constraints:

• 1 <= N <= 10^5
• S consists only of lowercase English letters.

Where S denotes the input string and N is its length.

Note: You do not need to print anything; implement the function as per the requirements.