Given an $m \times n$ integer matrix mat, return a list containing all elements of mat traversed in diagonal order.

In diagonal traversal, the diagonals alternate between moving upward-right and downward-left across the matrix, visiting every element exactly once.

Constraints:

• m $==$ mat.length

• n $==$ mat[i].length

• $1 \leq$ m, n $\leq 10^4$

• $1 \leq$ m $*$ n $\leq 10^4$

• ...