Complete String Problem Statement

Given an array of strings A of size N, determine the longest complete string. A string is deemed complete if every prefix of the string also appears in the array. If multiple strings of the same maximal length exist, return the lexicographically smallest one. If no complete string exists, return "None".

Input:

First line contains an integer 'T' denoting the number of test cases. Each test case includes: 
 An integer N indicating the size of array A.
 A list of N space-separated strings as elements of A.

Output:

For each test case, output the longest complete string in 'A'. If multiple strings of the same length exist, return the smallest lexicographically. If no string qualifies, return "None".

Example:

N = 4
A = [ "ab" , "abc" , "a" , "bp" ]

Explanation:
Prefixes of string "a" is “a” which is in 'A'.
Prefixes of "ab" are "a", "ab", both in 'A'.
"bp" has prefix "b" not in 'A'.
"abc" has prefixes "a", "ab", "abc", all in 'A'.
Thus, "abc" is the longest complete string.

Constraints:

• 1 ≤ T ≤ 10
• 1 ≤ N ≤ 100000
• 1 ≤ A[i].length ≤ 100000
• The characters in A[i] are lowercase English letters.
• Sum of all A[i].length across test cases ≤ 100000

Note:

No need to print anything, just implement the function to get the output.