[LeetCode]--43. 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.
Converting the input string to integer is NOT allowed.
You should NOT use internal library such as BigInteger.

字符串拆分成字符，然后计算，注意进位就行，进位中又要特别注意最高位进位。最开始我用int计算，其实还是逃不过超出范围的限制，所以就用个int的数组，只记录每一位数字，然后用这个数组来构造字符串。

public class Solution {
    public String multiply(String num1, String num2) {
        int[] res = new int[num1.length() + num2.length()];
        int temp = 0, flag = 0, i = num1.length() - 1, j = 0;
        for (; i >= 0; i--) {
            for (j = num2.length() - 1; j >= 0; j--) {
                temp = res[i + j + 1] + (num1.charAt(i) - '0')
                        * (num2.charAt(j) - '0') + flag;
                flag = temp / 10;
                res[i + j + 1] = temp % 10;
            }
            if (flag != 0) {
                res[i + j + 1] = flag;
                flag = 0;
            }
        }
        StringBuilder sb = new StringBuilder();
        i = 0;

        while (i < res.length - 1 && res[i] == 0) {
            i++;
        }

        while (i < res.length) {
            sb.append(res[i++]);
        }
        return sb.toString();
    }
}

还有一个比我简便一点点方法。

public String multiply2(String num1, String num2) {
        if (num1 == null || num2 == null) {
            return null;
        }

        int len1 = num1.length(), len2 = num2.length();
        int len3 = len1 + len2;
        int i, j, product, carry;

        int[] num3 = new int[len3];
        for (i = len1 - 1; i >= 0; i--) {
            carry = 0;
            for (j = len2 - 1; j >= 0; j--) {
                product = carry + num3[i + j + 1]
                        + Character.getNumericValue(num1.charAt(i))
                        * Character.getNumericValue(num2.charAt(j));
                num3[i + j + 1] = product % 10;
                carry = product / 10;
            }
            num3[i + j + 1] = carry;
        }

        StringBuilder sb = new StringBuilder();
        i = 0;

        while (i < len3 - 1 && num3[i] == 0) {
            i++;
        }

        while (i < len3) {
            sb.append(num3[i++]);
        }

        return sb.toString();
    }