Get the first unique character of a string. Python - single traversal solution

python27.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-

from collections import OrderedDict


def find_index_of_first_unique_char(string):
    seen_once = OrderedDict()                       # LinkedHashMap
    seen_many = set()                               # HashSet
    for index, char in enumerate(string):           # O(N)
        if char not in seen_many:                   # O(1)
            if char in seen_once:                   # O(1)
                seen_many.add(char)                 # O(1)
                seen_once.pop(char)                 # O(1)
            else:
                seen_once[char] = index             # O(1)
    if not seen_once:                               # O(1)
        return -1
    else:
        return seen_once.popitem(last=False)[1]     # O(1)


assert find_index_of_first_unique_char("") == -1
assert find_index_of_first_unique_char("aa") == -1
assert find_index_of_first_unique_char("baa") == 0
assert find_index_of_first_unique_char("zabcddd") == 0
assert find_index_of_first_unique_char("this is my string") == 1
assert find_index_of_first_unique_char("this this is my string") == 13
assert find_index_of_first_unique_char("this this is my string add an m") == 14

python37.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-

def find_index_of_first_unique_char(string):
    seen_once = dict()                          # LinkedHashMap
    seen_many = set()                           # HashSet
    for index, char in enumerate(string):       # O(N)
        if char not in seen_many:               # O(1)
            if char in seen_once:               # O(1)
                seen_many.add(char)             # O(1)
                seen_once.pop(char)             # O(1)
            else:
                seen_once[char] = index         # O(1)
    if not seen_once:                           # O(1)
        return -1
    else:
        for char in seen_once:                  # O(1) γιατί επιστρέφουμε στη πρώτη επανάληψη
            return seen_once[char]


assert find_index_of_first_unique_char("") == -1
assert find_index_of_first_unique_char("aa") == -1
assert find_index_of_first_unique_char("baa") == 0
assert find_index_of_first_unique_char("zabcddd") == 0
assert find_index_of_first_unique_char("this is my string") == 1
assert find_index_of_first_unique_char("this this is my string") == 13
assert find_index_of_first_unique_char("this this is my string add an m") == 14