大数算法

网上代码：实现两个超大数值的相乘

public class MyMultiply {  

    /** 
     * @param args 
     */  
    public static void main(String[] args) {  
        // System.out.println("Hello world");  

        //String str1 = "1234";  
        //String str2 = "1234";  
        //String str1 = "1076060999";  
        //String str2 = "90188905567";  
        String str1 = "23456789009877666555544444";  
        String str2 = "346587436598437594375943875943875";  

        int len1 = str1.length();  
        int len2 = str2.length();  

        char[] s1 = str1.toCharArray();  
        char[] s2 = str2.toCharArray();  

        // 高低位对调  
        covertdata(s1, len1);  
        covertdata(s2, len2);  

        System.out.println("乘数："+str1);  
        System.out.println("乘数："+str2);  
        multiply(s1, len1, s2, len2);  

    }  

    public static void covertdata(char data[], int len) {  
        //高低位对调  
        for (int i = 0; i < len / 2; i++) {  
            data[i] += data[len - 1 - i];  
            data[len - 1 - i] = (char) (data[i] - data[len - 1 - i]);  
            data[i] = (char) (data[i] - data[len - 1 - i]);  
        }  
    }  

    public static void multiply(char a[], int alen, char b[], int blen) {  
        // 两数乘积位数不会超过乘数位数和+ 3位  
        int csize = alen + blen + 3;  
        // 开辟乘积数组  
        int[] c = new int[csize];  
        // 乘积数组填充0  
        for (int ii = 0; ii < csize; ii++) {  
            c[ii] = 0;  
        }  
        // 对齐逐位相乘  
        for (int j = 0; j < blen; j++) {  
            for (int i = 0; i < alen; i++) {  
                c[i + j] +=  Integer.parseInt(String.valueOf(a[i]))* Integer.parseInt(String.valueOf(b[j]));  
            }  
        }  
        int m = 0;  
        // 进位处理  
        for (m = 0; m < csize; m++) {  
            int carry = c[m] / 10;  
            c[m] = c[m] % 10;  
            if (carry > 0)  
                c[m + 1] += carry;  
        }  
        // 找到最高位  
        for (m = csize - 1; m >= 0;) {  
            if (c[m] > 0)  
                break;  
            m--;  
        }  
        // 由最高位开始打印乘积  
        System.out.print("乘积：");  
        for (int n = 0; n <= m; n++) {  
            System.out.print(c[m - n]);  
        }  
        System.out.println("");  
    }  
}