As can be seen from the figure, odd rows are arranged from left to right, and even rows are arranged from right to left. 1000 is the 999th number, and 999/4=249.75, so 1000 is in line 250th.

249*4=996, and the last number in line 249 is 996- 1=997.

So the last two lines are arranged in this order:

994 995 996 997

1000 999 998