Recently, I began to review data structure and algorithms. Basicly I followed cuifenghui's steps. Yesterday I viewed his Big Integers Multiplication programs. Well, the idea is easy and great: use two char arrays to store two big numbers, then multiply them byte by byte. The following is my java version of his code.
 1 import java.io.*;
 2 
 3 public class Main {
 4     
 5     public Main() {
 6     }
 7 
 8     public static void main(String [] args) {
 9         String op1 = "", op2 = ""; // use String to store operators
10         char [] p1, p2, p; // use char array to compute
11 
12         System.out.println("\nThis program compute multiplication of two big integers.\n\n" +
13                 "Please input two big integers:");
14         try {
15             BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
16             op1 = br.readLine();
17             op2 = br.readLine();
18         } catch (IOException ioe) {
19         }
20 
21         p1 = op1.toCharArray();
22         p2 = op2.toCharArray();
23         int length = p1.length + p2.length;
24         p = new char[length];
25         for (int i = 0; i < length; i ++) { // initial the result
26             p[i] = '0';
27         }
28         
29         // the following is compute the result byte by byte
30         short carry = 0;
31         for (int i = p1.length - 1; i >= 0; i --) {
32             carry = 0;
33             for (int j = p2.length - 1; j >= 0; j --) {
34                 carry += (p1[i] - '0') * (p2[j] - '0') + (p[i+j+1] - '0');
35                 p[i+j+1] = (char)(carry % 10 + '0');
36                 carry /= 10;
37             }
38             p[i] = (char)(carry + '0');
39         }
40 
41         boolean printing = false;
42         System.out.println("\nThe result is:");
43         for (int i = 0; i < p.length; i ++) {
44             if (!printing && p[i] == '0') // trim beginning 0
45                 continue;
46             System.out.print(p[i]);
47             printing = true;
48         }
49         System.out.println();
50     }
51 
52 }
53