大数的乘法

求两个很大的数相乘，如：123456789123456789*123456789123456789

 

package com.ldq;

/*
 * 两个很大的数相乘
 * */

public class Multi {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		// TODO Auto-generated method stub

		String a = "123456789123456789123456789123456789123456789123456789";
		String b = "123456789123456789123456789123456789123456789123456789";

		int m = a.length();
		int n = a.length();

		int aa[] = new int[100];
		int bb[] = new int[100];
		int cc[] = new int[500];

		for (int i = 0; i < a.length(); i++) {
			aa[i] = Integer.parseInt(a.substring(a.length() - i - 1, a.length()
					- i));
			bb[i] = Integer.parseInt(b.substring(b.length() - i - 1, b.length()
					- i));
		}

		int t = 0;
		for (int j = 0; j < n; j++) {
			for (int i = 0; i < m; i++) {
				t = cc[i + j] + aa[i] * bb[j];
				cc[i + j] = t % 10;
				cc[i + j + 1] += t / 10;
				if (cc[i + j + 1] > 10) {
					cc[i + j + 1] %= 10;
					cc[i + j + 2]++;
				}
			}

		}
		for (int i = 200; i >= 0; i--) {
			System.out.print(cc[i]);
		}

	}
}

  

主要思路就是用两个整型数组保存相乘的数，用两个 for 循环模拟乘法过程就 OK 了。也不难。