def flatten_list(nested_list):
    flat_list = []
    stack = [nested_list]
    
    while stack:
        current = stack.pop()
        if isinstance(current, list):
            stack.extend(reversed(current))
        else:
            flat_list.append(current)
    
    return flat_list


def char_count(s: str):
    if not s:
        return {}
    char_dict = {}
    for char in s:
        char_dict[char] = char_dict.get(char, 0) + 1
    return char_dict
    
if __name__ == "__main__":
    nested_list = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
    print(flatten_list(nested_list))
    s = "hello world"
    print(char_count(s))