//1/1 1/2 2/1 3/1 2/2 1/3 1/4 2/3 3/2 4/1 1/5 2/4 ... //1 2 6 7 ... //3 5 8 ... //4 9 ... //10 ... //1/1 //1/2 2/1 //3/1 2/2 1/3 //1/4 2/3 3/2 4/1