>>912

n = 25; lis = {};
While[n > 0, m = Mod[n, 3];
lis = Prepend[lis, If[m != 2, m, -1]];
n = If[m != 2, Quotient[n, 3], Quotient[n, 3] + 1]];
Drop[lis, 1]

でどうですか?n=1000でも試してみて下さい