Problem Description

Find the smallest substring that contains all unique characters present in the input string. This is a variation of the minimum window substring problem where the window must contain all distinct characters from the original string.

Input Format

A single string containing lowercase letters.

Output Format

The smallest substring containing all unique characters. If multiple substrings have the same minimum length, return the first one found.

Constraints

• 1 ≤ string length ≤ 10^5

• String contains only lowercase English letters

• At least one character in the string

• Return the lexicographically first substring if multiple solutions exist

Sample Input/Output

aabcbcdbca

dbca

Explanation

The string contains unique characters: a, b, c, d. The smallest substring containing all these characters is "dbca" (length 4). Other valid substrings like "abcbcd" are longer.