Traverse the matrix diagonally and return the elements in traversal order.
Example 1
Input: mat = [[1,2,3],[4,5,6],[7,8,9]]
Output: [1,2,4,7,5,3,6,8,9]
Explanation: Traversal along diagonals from top to bottom.
Output: [1,2,4,7,5,3,6,8,9]
Explanation: Traversal along diagonals from top to bottom.
Example 2
Input: mat = [[1,2],[3,4]]
Output: [1,2,3,4]
Explanation: Diagonal traversal of a 2x2 matrix.
Output: [1,2,3,4]
Explanation: Diagonal traversal of a 2x2 matrix.