First Non-Repeating Character

Given a string, find the first character that does not repeat. If every character appears more than once or the string is empty, output an empty string.

For example, given the string swiss, the first non-repeating character is w. Use standard input to read a string and standard output to print the answer.

Note: You should implement a solution with optimal runtime performance. The algorithm should ideally perform in linear time, i.e., \(O(n)\), where \(n\) is the length of the string.

inputFormat

The input consists of a single line containing the string \(s\). The string may contain letters, digits, and/or special characters.

outputFormat

Output a single line containing the first non-repeating character of the input string. If no such character exists, output an empty string.

## sample

swiss

w

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