[[1,2,3,4],[5,6,7,8],[9,10,11,12]]
[1,5,9,10,11,12,8,4,3,2,6,7]
[[1,2,3],[4,5,6],[7,8,9]]
[1,4,7,8,9,6,3,2,5]
m == matrix.lengthn == matrix[i].length1 0