I think b/c it's using the difference table method in both cases. Where as, if the pattern starts with 1, it recognizes it as powers of 2. R code: solver

I think b/c it's using the difference table method in both cases. Where as, if the pattern starts with 1, it recognizes it as powers of 2. R code: solver <- function(x) { print(x) if (length(x) == 0 || all(x == 0)) 0 else x[length(x)] + solver(diff(x)) } solver(c(0,1,2,4,8,16,32,64,128)) #=> 255 solver(c(0,1,2,4,8,16,32,64,128,256)) #=> 512 solver(c(0,1,2,4,8,16,32,64,128,256,512)) #=> 1023 solver(c(0,1,2,4,8,16,32,64,128,26,512,1024)) #=> 2048