由ab < 0 说明他们异号.
设 a > 0 b< 0 ==> a = 6 b = -7 ==> a+b = -1 ==> 立方根=-1
设 a 0 ==> a=-6 b=7 ==> a+b = 1 ==> 立方根=1
所以 其立方根是1或者-1