how to use python to find the first not repeating character?

I am solving a problem: Given a string s consisting of small English letters, find and return the first instance of a non-repeating character in it. If there is no such character, return ‘_’.

For example: s = “abacabad”, the output should be firstNotRepeatingCharacter(s) = ‘c’.

I wrote a simple code, it got through all the test, but when I submit it, it reports error, anyone know what’s wrong with my code? Thank you!

def firstNotRepeatingCharacter(s):   
    for i in list(s):
        if list(s).count(i) == 1:
            return i
    return '_'

Answer

Could be a performance issue as your repeated count (and unnecessary list conversions) calls make this approach quadratic. You should aim for a linear solution:

from collections import Counter

def firstNotRepeatingCharacter(s):   
    c = Counter(s)
    for i in s:
        if c[i] == 1:
            return i
    return '_'

You can also use next with a generator and a default value:

def firstNotRepeatingCharacter(s):   
    c = Counter(s)
    return next((i for i in s if c[i] == 1), '_')

If you can only use built-ins, just make your own counter (or any other data structure that allows you to identify duplicates)

def firstNotRepeatingCharacter(s):   
    c = {}
    for i in s:
        c[i] = c.get(i, 0) + 1
    return next((i for i in s if c[i] == 1), '_')