字符串相乘

求解代码

public String multiply(String num1, String num2) {
            if ("0".equals(num1) || "0".equals(num2)) {
                return "0";
            }

            int len1 = num1.length();
            int len2 = num2.length();
            int[] res = new int[len1 + len2];

            // 从后往前遍历
            for (int i = len1 - 1; i >= 0; i--) {
                int n1 = num1.charAt(i) - '0';
                for (int j = len2 - 1; j >= 0; j--) {
                    int n2 = num2.charAt(j) - '0';
                    int sum = n1 * n2 + res[i + j + 1]; // 加上原来的低位
                    res[i + j + 1] = sum % 10;
                    res[i + j] += sum / 10;
                }
            }

            StringBuilder sb = new StringBuilder();
            for (int num : res) {
                if (!(sb.length() == 0 && num == 0)) {
                    sb.append(num);
                }
            }
            return sb.length() == 0 ? "0" : sb.toString();
        }