通过位运算把一个数的最高位设成所有位怎么做？

copyMSB - set all bits of result to most significant bit of x.

    Examples: copyMSB(0x80000000) = 0xFFFFFFFF
                                      copyMSB(0x70000000) = 0x00000000
      Legal ops: ! ~ & ^ | + << >>
      Max ops: 5

不能用if-else。

 unsigned int copyMSB(unsigned int x)
{
    return x >> 31 << 31;
}

int main()
{
    printf("%d\n", copyMSB(0x80000000));
    printf("%d\n", copyMSB(0x70000000));
}

 #include <stdio.h>
unsigned int copyMSB(unsigned int x)
{
    return ~( 0xFFFFFFFF+ (x>>31) );
}

int main()
{
    printf("%08X\n", copyMSB(0x80000000));
    printf("%08X\n", copyMSB(0x70000000));
}

有没有规定数据位数…………

这样也行：

 #include <stdio.h>
unsigned int copyMSB(int x)
{
    return x>>31 ;
}

int main()
{
    printf("%08X\n", copyMSB(0x80000000));
    printf("%08X\n", copyMSB(0x70000000));
}