Diagonal Traverse

Given an m x n matrix, return all elements in 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]