原式=(x^2+1)*(f(x)+1)-1
要使原式为奇数,则需x为奇数或(x为偶数且f(x)为奇数)
故1可映射4,5,6
2可映射5
3可映射4,5,6
由乘法原理,总方案数3*1*3=9
再者说,穷举一下也行啊(f(1),f(2),f(3))
(4,5,4)(4,5,5)(4,5,6)
(5,5,4)(5,5,5)(5,5,6)
(6,5,4)(6,5,5)(6,5,6)
共9种