大数的阶乘

  前一段时间swing & swt群里一个大牛叫我用自己的方法写个大数的阶乘，前一段时间抽了一点时间，写出来了，效率不太好，现在贴出来共享下，希望csdn中的大牛们来给我优化优化，提出更优美的算法，恶哈哈！

 pckage org.loonsoft.test;import java.util.Scanner;public class BigIntegerFac {/** * 处理大数相加 * * @param a * @param b * @return a+b结果 */private static StringBuilder add(String a, String b) {StringBuilder sb = new StringBuilder();int len1 = a.length();int len2 = b.length();if (len1 > len2) {String temp = a;a = b;b = temp;int len = len1;len1 = len2;len2 = len;}int c = 0; // 进位for (int i = a.length() - 1; i >= 0; --i) {int aa = Integer.parseInt(String.valueOf(a.charAt(i)));int bb = Integer.parseInt(String.valueOf(b.charAt(len2 - 1)));int cc = aa + bb + c;if (cc >= 10) {c = cc / 10;sb.append(cc % 10);} else {sb.append(cc);c = 0;}--len2;}for (int i = len2 - 1; i >= 0; --i) {int aa = Integer.parseInt(String.valueOf(b.charAt(i)));int bb = c;int cc = aa + bb;if (cc >= 10) {c = cc / 10;sb.append(cc % 10);} else {sb.append(cc);c = 0;}}if (c > 0)sb.append(c);return sb.reverse();}/** * 处理大数相乘 * * @param a * @param b * @return a*b */private static String mutiply(String a, String b) {StringBuilder ans1 = new StringBuilder();StringBuilder ans2 = new StringBuilder();int len1 = a.length();int len2 = b.length();int r = 1;// 控制做加法的时候的缩进// System.out.println(len1 + " " + len2);for (int i = len2 - 1; i >= 0; --i) {int bb = Integer.parseInt(String.valueOf(b.charAt(i)));int c = 0; // 保存进位for (int j = len1 - 1; j >= 0; --j) {int aa = Integer.parseInt(String.valueOf(a.charAt(j)));int cc = aa * bb + c;// System.out.println(cc);if (cc >= 10) {c = cc / 10;ans1.append(cc % 10);} else {ans1.append(cc);c = 0;}}if (c > 0)ans1.append(c);if (i == len2 - 1) {ans2 = ans2.append(ans1.reverse());} else {StringBuilder temp = ans1.reverse();int tmpR = r;while (tmpR > 0) {temp.append(0);--tmpR;}++r;ans2 = add(temp.toString(), ans2.toString());}ans1.delete(0, ans1.length());}return ans2.toString();}/** * 求阶乘 * * @param n * @return n! */public static String getFac(int n) {String fac = "1";for (int i = 1; i <= n; ++i) {fac = mutiply(fac, String.valueOf(i));}return fac.toString();}public static void main(String[] args) {Scanner sc = new Scanner(System.in);while (sc.hasNext()) {int n = sc.nextInt();// long b = System.currentTimeMillis();System.out.println(getFac(n));// System.out.println(System.currentTimeMillis() - b);}}}