-1/20=?
C++ 里面结果是 0
Python 里面结果是 -1
Python里面的注意事项里面，也提到了这点“总是向负无穷取整”
For (plain or long) integer division, the result is an integer. The result is always rounded towards minus infinity: 1/2 is 0, (-1)/2 is -1, 1/(-2) is -1, and (-1)/(-2) is 0. Note that the result is a long integer if either operand is a long integer, regardless of the numeric value.
C++ 里面也想得到这样的结果，就必须把整形的除法转换为浮点数的除法，然后对结果取 floor。
static_cast<int>(floor(static_cast<double>(-1)/20) )