编写函数实现：clear all but least signficant n bits of x

方法1：

int lowe_bits(int x, int n){    int w = sizeof(int) << 3;    unsigned y = ~0;    y >>= (w - n);    return y & x;    }

方法2：

int lower_bits(int x, int n){    int mask = ~(0 - (1 << n - 1) -( 1 << n - 1));    return mask & x;}