Function find_diagonal_order

pub fn find_diagonal_order(matrix: Vec<Vec<i32>>) -> Vec<i32>

When meet following conditions:

1. up and j == n-1: i++
2. up and i == 0: j++
3. down and j == m-1: j++
4. down and j == 0: i++ and let up = !up otherwise go up and right or go down and left.