What pattern does this problem fall into?
Given an m x n matrix mat, return an array of all the elements of the array in a diagonal order.
Example 1:
Input: mat = [[1,2,3],[4,5,6],[7,8,9]]
Output: [1,2,4,7,5,3,6,8,9]
Example 2:
Input: mat = [[1,2],[3,4]]
Output: [1,2,3,4]