7个
f(a)=f(b)+f(c)
当f(a)=0
1.f(b)=-1 f(c)=1
2.f(b)=1 f(c)=-1
3.f(b)=0 f(c)=0
当f(a)=1
f(b)=0 f(c)=1
f(b)=1 f(c)=0
当f(a)=-1
f(b)=-1 f(c)=0
f(b)=0 f(c)=-1
共7种