We use a hash map (or fixed-size array) to count the frequency of each character. Then, we re-traverse the string to find the first character with a frequency of 1.

1. Count character frequencies in s using a Counter.
2. Iterate through s using its index i.
3. If count[s[i]] == 1, return i.
4. If loop finishes, return -1.

• Time Complexity: O(N).
• Space Complexity: O(1) (since the alphabet size is constant).

from collections import Counter

def first_uniq_char(s):
    counts = Counter(s)
    for i, char in enumerate(s):
        if counts[char] == 1:
            return i
    return -1