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();

		// 高低位對調
		covertdata(s1, len1);
		covertdata(s2, len2);

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

	}

	public static void covertdata(char data[], int len) {
		//高低位對調
		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) {
		// 兩數乘積位數不會超過乘數位數和+ 3位
		int csize = alen + blen + 3;
		// 開闢乘積陣列
		int[] c = new int[csize];
		// 乘積陣列填充0
		for (int ii = 0; ii < csize; ii++) {
			c[ii] = 0;
		}
		// 對齊逐位相乘
		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;
		// 進位處理
		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--;
		}
		// 由最高位開始列印乘積
		System.out.print("乘積：");
		for (int n = 0; n <= m; n++) {
			System.out.print(c[m - n]);
		}
		System.out.println("");
	}
}