字符串相乘

public class T43 {
    public String multiply(String num1, String num2) {
        if (num1 == null || num1.length() == 0  || num2 == null || num2.length() == 0) {
            return null;
        }
        if (num1.equals("0") || num2.equals("0")) {
            return "0";
        }
        int len1 = num1.length();
        int len2 = num2.length();
        int len3 = len1 + len2;

        int[] num3 = new int[len3];
        for (int i = len1 - 1; i >= 0; i--) {
            for (int j = len2 - 1; j  >= 0; j--) {
                //注意累加——同是此位上的值，i为2j为1 和 i为1 j为2 时均会加到此位置上
                num3[i + j + 1] += (num1.charAt(i) - ‘0‘) * (num2.charAt(j) - ‘0‘);
            }
        }
        int carry = 0;
        for (int i = num3.length - 1; i >= 0; i--) {
            int temp = (num3[i] + carry) % 10;
            carry = (num3[i] + carry) / 10;
            num3[i] = temp;
        }
        int i = 0;
        String res = "";
        while (num3[i] == 0) {
            i++;
        }
        for (int j = i; j < num3.length; j++) {
            res += num3[i];
        }
        return res;
    }
}