1. 觀察矩陣對角線模式
   1. (0,0) -> 1 -> i+j = 0
   2. (1, 0), (0, 1) -> 2 -> i+j = 1
   3. (2, 0), (1, 1), (0, 2) -> 3 -> i+j = 2
2. how to deal with diagonal sorting?
   1. while (i+j) % 2 == 1 
   2. while (i+j) % 2 == 0 reversed