Problem

Given a list of numbers, write a Python program to find the first unique element in the list.
Example input: [1, 2, 3, 4, 5, 1, 2, 3, 4]
Example output: 5

Solution

# Solution:
# The solution below is optimal because it uses a dictionary to store the number of times each element appears in the list.
# The dictionary is then iterated through to find the first element that appears only once.
# This solution is optimal because it only iterates through the list once and then iterates through the dictionary once.
# This is optimal because the dictionary is a constant size and the list is the only variable size.
# Therefore, the time complexity is O(n) where n is the size of the list.

def find_first_unique(list):
    dict = {}
    for i in list:
        if i in dict:
            dict[i] += 1
        else:
            dict[i] = 1
    for i in list:
        if dict[i] == 1:
            return i
    return None

print(find_first_unique([1, 2, 3, 4, 5, 1, 2, 3, 4]))