Nth Non-Repeating Character

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Your question is Nth Non-Repeating Character. Start with the requirements on the right.

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Problem

A Zynga game event stream contains a sequence of characters representing compact player actions. Given a string s and a positive integer n, return the nth character that appears exactly once in the entire string, preserving the characters' original order.

Process the input string in a single left-to-right iteration. If fewer than n characters are non-repeating, return None.

Formal Specification

Input: s, a string, and n, a positive integer.
Output: The nth non-repeating character as a one-character string, or None if it does not exist.
Character comparisons are case-sensitive, so A and a are different characters.

Constraints

0 <= len(s) <= 10^5
1 <= n <= 10^5
Character comparisons are case-sensitive
The input string must be traversed only once

Function Signature

def find_nth_non_repeating(s, n):

Your solutionPython 3

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