First character that appears only once

The first character that appears only once

In string s Find the first character that appears only once . without , Return a single space . s Contains only lowercase letters .
Example :
s = “abaccdeff”
return “b”
s = “”
return " "
Ideas 1 explain ： String rotation list, Count the number of occurrences of each character , If the number of occurrences of a character is 1, Output , Otherwise, the output is empty . This idea increases with the length of the string , It takes a lot of time .

Ideas 2 explain ： Build a dictionary , The character appears once as True, Appear many times , by False, One of the output dictionaries True Corresponding characters