LeetCode 43 - 大数乘法

模拟算乘法，实现大整数乘法。仅限整数，负数要加点判断条件。

这个方法虽然不是最快的，但比较好理解。

class Solution {
    public String multiply(String num1, String num2) {
        // trivial case
        if (num1.equals("0") || num2.equals("0")) {
            return "0";
        }
        if (num1.equals("1"))
            return num2;
        if (num2.equals("1"))
            return num1;

        // method call optimition
        int len1 = num1.length();
        int len2 = num2.length();

        // target result;
        int[] res = new int[len1 + len2];

        for (int i = len1 - 1; i >= 0; i--) {
            for (int j = len2 - 1; j >= 0; j--) {
                // multiply single numbers
                int tmp = (num1.charAt(i) - '0') * (num2.charAt(j) - '0');
                // len - i - 1 means indentation
                // add up two single num
                res[(len1 - i - 1) + len2 - j - 1] += tmp % 10;

                // carry
                int add = res[(len1 - i - 1) + len2 - j - 1] / 10;
                res[(len1 - i - 1) + len2 - j - 1] %= 10;
                res[(len1 - i - 1) + len2 - j] += add;
                res[(len1 - i - 1) + len2 - j] += tmp / 10;
            }
        }
        int fin = res.length - 1;
        while (res[fin] == 0)
            fin--;
        StringBuilder sb = new StringBuilder();
        // reverse order to append as string
        while (fin >= 0) {
            sb.append((char) (res[fin--] + '0'));
        }
        return sb.toString();
    }
}