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]
Constraints:
m == mat.lengthn == mat[i].length1 <= m, n <= 1041 <= m * n <= 104-105 <= mat[i][j] <= 105