Problem

Given a string s, return the first character that appears exactly once when scanning from left to right. If no such character exists, return an empty string "". The input is a single string, and the output is a single character string or an empty string.

Examples

Example 1:

Input: s = "leetcode"
Output: "l"
Explanation: 'l' is the first character with frequency 1.

Example 2:

Input: s = "aabb"
Output: ""
Explanation: Every character appears more than once.

Constraints

• 0 <= len(s) <= 10^5
• s contains printable characters
• Return the first non-repeating character in original order, not just any unique character