大数算法(大数相加)

这几天做projecteuler
发现数学很重要啊！
当计算非常大的数的相乘时，使用BigDecimal
便希望能自己实现大数的乘法
因为乘法里要使用加法，就先实现加法了
开始时，大数使用字符串保存
这时候我们需要将大数存储在一个数组里
为了节省空间，我们使用Byte存储每一位

	public static Byte[] StringToByte(String number){
		int len = number.length();
		Byte[] result = new Byte[len];
		//从高位到低位依次转换,转换后字符串"1234"变为4321,这样方便以后计算
		for(int i=0; i<number.length(); i++){
			byte curByte =  Integer.valueOf(number.charAt(i)+"").byteValue();
			result[len-i-1] = curByte;
		}
		
		return result;
	}

加法算法

	public static Byte[] Add(Byte[] number1, Byte[] number2){
		int len = (number1.length>number2.length) ? number1.length : number2.length;
		Byte[] result = new Byte[len+1];
		Integer sum = new Integer(0);
		Integer carry = new Integer(0);
		int i;
		if(number1.length>number2.length){
			for(i=0; i<number2.length; i++){
				sum = number1[i].intValue() + number2[i].intValue();				
				sum = sum +carry;
				carry = ((sum -10)>=0) ? 1:0;
				sum = ((sum -10)>=0) ? (sum-10):sum;
				result[i] = sum.byteValue();
				sum = 0;
			}
			for(;i<number1.length;i++){
				sum = sum +carry;
				carry = ((sum -10)>=0) ? 1:0;
				sum = ((sum -10)>=0) ? (sum-10):sum;
				result[i] = sum.byteValue();
				sum = 0;
			}
			if(carry!=0){
				result[i] = carry.byteValue();
			}else{
				result[i] = 0;
			}
		}else{
			for(i=0; i<number1.length; i++){
				sum = number1[i].intValue() + number2[i].intValue();				
				sum = sum +carry;
				carry = ((sum -10)>=0) ? 1:0;
				sum = ((sum -10)>=0) ? (sum-10):sum;
				result[i] = sum.byteValue();
				sum = 0;
			}
			for(;i<number2.length;i++){
				sum = number2[i].intValue() + carry;
				carry = ((sum -10)>=0) ? 1:0;
				sum = ((sum -10)>=0) ? (sum-10):sum;
				result[i] = sum.byteValue();
				sum = 0;
			}
			if(carry!=0){
				result[i] = carry.byteValue();
			}else{
				result[i] = 0;
			}
		}
		
		return result;
	}