用位运算实现加减乘除

        //加法
	public static int add(int a, int b)  
	{  
	    int ans;  
	    while(b!=0)  
	    {
	        ans = a^b;
	        b = ((a&b)<<1);
	        a = ans;  
	    }  
	    return a;  
	}  
	
	//减法
	public static int sub(int a, int b)
	{  
	    return add(a, -b);  
	}
	
	//正数乘法
	private static int posMultiply(int a,int b)  
	{  
	    int ans = 0;  
	    while(b>0)  
	    {  
	        if((b&0x1)==1)ans = add(ans, a);  
	        a = (a<<1);  
	        b = (b>>1);  
	    }  
	    return ans;  
	}  
	  
	//乘法
	public static int multiply(int a,int b)  
	{  
	    if(a==0||b==0)return 0;  
	    if(a>0 && b>0)  
	        return posMultiply(a, b);  
	    if(a<0)  
	    {  
	        if(b<0)  
	        {  
	            return posMultiply(-a, -b);  
	        } 
	        else
	        {
	        	return -posMultiply(-a, b ); 
	        }
	    }
	    else
	    {
	    	return -posMultiply(a, -b);
	    }
	}  
	  
	//正数除法
	private static int posDiv(int x,int y)  
	{  
	    int ans=0;  
	    for(int i=31;i>=0;i--)  
	    {
	        if((x>>i)>=y)  
	        {  
	            ans+=(1<<i);  
	            x-=(y<<i);  
	        }  
	    }  
	    return ans;  
	}  
	  
	//除法
	public static int div( int a, int b )  
	{
		assert(b!=0);
	    if(a==0)return 0;

	    if(a>0)  
	    {  
	        if(b>0)
	        {
	            return posDiv(a,b); 
	        }
	        else
	        {
	        	return -posDiv( a,-b);  
	        }
	    }  
	    else
	    {
	    	if(b>0)
	    	{
		        return -posDiv(a, b);  
	    	}
	    	else
	    	{
	    		return -posDiv(-a, -b);  
	    	}
	    }
	    
	}   
	
	//求负数
	public static int negtive(int a)
	{  
	    return add(~a, 1);  
	}
	  
	//比较正数大小
	private static boolean isbigerPos( int a, int b )   
	{
	    int c = 1;  
	    b = (a^b);  
	    if(b==0)return false;
	    
	    while(b>0)  
	    {  
	    	b>>=1;
	        c <<= 1;  
	    }  
	    return (c&a)==0;  
	}   
	  
	//比较大小   
	public static boolean isbiger( int a, int b )   
	{
	    if(a<0)  
	    {  
	        if(b<0)  
	        {  
	            return isbigerPos( negtive(b), negtive(a) );  
	        }
	        else
	        {
	        	return false;  
	        }
	    }
	    else
	    {
		    if(b<0)
		    {
		        return true;  
		    }
		    else
		    {
		    	return isbigerPos(a, b);  
		    }
	    }

	}

	public static int divideby3(int x)  
	{  
	    int sum = 0;  
	    while(x > 3)  
	    {  
	        sum = add(x>>2 , sum);  
	        x = add(x>>2 , x&3);  
	    } 
	    if(x == 3) 
	    {
	        sum = add(sum , 1);
	    }
	    return sum;  
	}  
	
	public static void main(String[] args)
	{
		System.out.println(add(13,19));
		System.out.println(sub(13,19));
		System.out.println(multiply(13,19));
		System.out.println(div(10,5));
		System.out.println(negtive(13));
		System.out.println(isbiger(10,5));
		System.out.println(isbiger(10,11));
		
		System.out.println(divideby3(15));
		System.out.println(divideby3(14));
		System.out.println(divideby3(13));
		System.out.println(divideby3(12));
		System.out.println(divideby3(11));
	}