For a positive integer n, return an nXn matrix mat such that the value of each element in row i and column j is given according to the following rules:

• i - j, if i > j
• j - i, if i < j
• 0, if i equals j

If n is not a positive integer, mat should be an empty matrix.

Examples:

Input:  n   = 4
Output: mat = [0  1  2  3
               1  0  1  2
               2  1  0  1
               3  2  1  0]

Input:  n   = -2
Output: mat = []

Input:  n   = 2.5
Output: mat = []