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public class Solution {
    public String addStrings(String num1, String num2) {
        if (num1 == null || num2 == null) return null;
        int len1 = num1.length();
        int len2 = num2.length();
        if (len1 == 0) {
            return num2;
        } else if (len2 == 0) {
            return num1;
        }
        int sum = 0;
        StringBuilder res = new StringBuilder();
        while(len1 > 0 || len2 > 0) {
            if (len1 > 0) sum += num1.charAt(--len1) - '0';
            if (len2 > 0) sum += num2.charAt(--len2) - '0';
            res.append(sum % 10);
            sum /= 10;
        }
        res.append(sum == 0 ? "" : sum);
        return res.reverse().toString();
    }
}