比较函数

\[ax < b \,\, ==> \,\, x <= (b + a - 1) / a - 1 \]

\[ax <= b \,\, ==> \,\, x <= b / a \]

\[ax > b \,\, ==> \,\, x >= b / a + 1 \]

\[ax >= b \,\, ==> \,\, x <= (b + a - 1) / a \]

//处理后均符合c++的向下取整标准