Ninja and Alien Language Order Problem

An alien dropped its dictionary while visiting Earth. The Ninja wants to determine the order of characters used in the alien language, based on the given list of words from the alien's dictionary.

Example:

Input:

words = ["baa", "abcd", "abca", "cab", "cad"]

Output:

Order: "bdac"

Explanation:

The order of characters is derived from the following constraints based on the words' order:
"baa" before "abcd": 'b' comes before 'a'.
"abcd" before "abca": 'd' comes before 'a'.
Resulting in the order 'b', 'd', 'a', 'c'.

Constraints:

• The list of words may have more than one valid order; return the smallest lexicographical order.
• Return an empty string if there is no valid order.
• 1 <= T <= 10
• 1 <= N <= 300
• 0 <= size of a word <= 100
• Time limit: 1 sec