Problem

Given a string, return the first character that is not repeated in the string.
Example input: "abcdab"
Example output: "c"

Solution

def first_non_repeated(string):
    
    This function takes a string and returns the first character that is not repeated in the string.
    
    # Create a dictionary to store the number of times each character appears in the string
    char_count = {}
    # Iterate through the string
    for char in string:
        # If the character is not in the dictionary, add it and set the value to 1
        if char not in char_count:
            char_count[char] = 1
        # If the character is in the dictionary, increment the value by 1
        else:
            char_count[char] += 1
    # Iterate through the string
    for char in string:
        # If the character appears only once in the string, return it
        if char_count[char] == 1:
            return char

# Test cases
print(first_non_repeated("abcdab"))
print(first_non_repeated("abcd"))
print(first_non_repeated("aabbccdd"))
print(first_non_repeated("abc defghijklmnopqrstuvwxyz"))
print(first_non_repeated("abcdefghijklmnopqrstuvwxyza"))
print(first_non_repeated("abcdefghijklmnopqrstuvwxyzz"))
print(first_non_repeated("abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyz"))
print(first_non_repeated("abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyza"))
print(first_non_repeated("abcdefghijklmnopqrstuvwxyzabcdefghijklmnopqrstuvwxyzz"))


This solution is optimal because it iterates through the string only once to create the dictionary and then iterates through the string again to find the first character that is not repeated.