First non-repeating in a stream

You are given an input stream s consisting of lowercase alphabets. As you read characters from the stream one by one, determine which character has appeared only once in the stream up to that point. If multiple characters have appeared only once, identify the first one that appeared. If no such character exists, append # to the result. Your task is to build a string that represents the character status for each point in the stream.

NOTE:

1. You must determine the result for each index i in the stream, where 0=i<n.
2. To compute the result for each index i, consider the substring starting from the first character up to the i-th character in the stream.