OPT02

class Solution {
    StringBuilder add(StringBuilder a, StringBuilder b) {
        int p1 = a.length() - 1;
        int p2 = b.length() - 1;
        int c = 0;
        StringBuilder sb = new StringBuilder();
        while (p1 >= 0 && p2 >= 0) {
            int d1 = a.charAt(p1) - '0';
            int d2 = b.charAt(p2) - '0';
            int s = (d1 + d2 + c);
            int d3 = (s % 10);
            c = s / 10;
            sb.append((char) (d3 + '0'));
            p1--;
            p2--;
        }
        while (p1 >= 0) {
            int d1 = a.charAt(p1) - '0';
            int s = (d1 + c);
            int d = s % 10;
            c = s / 10;
            sb.append((char) (d + '0'));
            p1--;
        }
        while (p2 >= 0) {
            int d1 = b.charAt(p2) - '0';
            int s = (d1 + c);
            int d = s % 10;
            c = s / 10;
            sb.append((char) (d + '0'));
            p2--;
        }
        if (c > 0) sb.append((char) (c + '0'));
        int sbLen = sb.length() - 1;
        while (sbLen > 0 && sb.charAt(sbLen) == '0') {
            sb.deleteCharAt(sbLen);
            sbLen--;
        }
        if (sb.length() == 0) return new StringBuilder("0");
        sb.reverse();
        return sb;
    }

    public String multiply(String num1, String num2) {
        StringBuilder ans = new StringBuilder("0");
        int pos = 0;
        for (int i = num1.length() - 1; i >= 0; i--) {
            StringBuilder currentResult = new StringBuilder();
            for (int x = 0; x < pos; x++) {
                currentResult.append("0");
            }
            int md = num1.charAt(i) - '0';
            int c = 0;
            for (int j = num2.length() - 1; j >= 0; j--) {
                int cd = num2.charAt(j) - '0';
                int s = (cd * md) + c;
                int ad = (s % 10); // ans digit
                c = s / 10;
                currentResult.append((char) (ad + '0'));
            }
            if (c > 0) {
                currentResult.append((char) (c + '0'));
            }
            int sbLen = currentResult.length() - 1;
            while (sbLen > 0 && currentResult.charAt(sbLen) == '0') {
                currentResult.deleteCharAt(sbLen);
                sbLen--;
            }
            if (currentResult.length() == 0) currentResult = new StringBuilder("0");
            currentResult.reverse();
            ans = add(ans, currentResult);
            pos++;
        }
        return ans.toString();
    }

}