Multiply Strings

Given two numbers represented as strings, return multiplication of the numbers as a string.

Note: The numbers can be arbitrarily large and are non-negative.

 1 public String multiply(String num1, String num2) {

 2         if(num1.equals("0")||num2.equals("0")){

 3             return "0";

 4         }

 5         String re = "";

 6         for (int i = 0; i < num1.length(); i++) {

 7             String temp = "";

 8             int in = 0;

 9             for (int j = 0; j < num2.length(); j++) {

10                 int k = (num2.charAt(num2.length() - 1 - j) - '0')

11                         * (num1.charAt(num1.length() - 1 - i) - '0') + in;

12                 temp = (k % 10) + temp;

13                 in = k / 10;

14             }

15             if (in > 0) {

16                 temp = in + temp;

17             }

18             if (i == 0) {

19                 re = temp;

20             } else {

21                 String t = "";

22                 int in2 = 0;

23                 for (int m = 1; m <= i; m++) {

24                     t = String.valueOf(re.charAt(re.length() - m)) + t;

25                 }

26                 for (int m = 0; m <= re.length() - 1 - i; m++) {

27                     int k = (temp.charAt(temp.length() - 1 - m) - '0')

28                             + (re.charAt(re.length() - 1 - i - m) - '0') + in2;

29                     t = (k % 10) + t;

30                     in2 = k / 10;

31                 }

32                 for (int m = 0; m < temp.length() + i - re.length(); m++) {

33                     int k = (temp.charAt(temp.length() + i - re.length() - m

34                             - 1) - '0')

35                             + in2;

36                     t = (k % 10) + t;

37                     in2 = k / 10;

38                 }

39                 if (in2 > 0) {

40                     t = in2 + t;

41                 }

42                 re = t;

43             }

44             // System.out.println(temp);

45         }

46         return re;

47     }

没啥可说的，去看BigInteger实现吧。。。

我自己写的耗时716ms，调用BigInteger耗时480ms。。