415. Add Strings 

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class Solution {
    public String addStrings(String num1, String num2) {
        int i = num1.length() - 1, j = num2.length() - 1;

        StringBuilder sb = new StringBuilder();

        int carry = 0;
        while (carry != 0 || i >= 0 || j >= 0) {
            int x = i >= 0 ? num1.charAt(i--) - '0' : 0;
            int y = j >= 0 ? num2.charAt(j--) - '0' : 0;
            int sum = carry + x + y;
            sb.append(sum % 10); // 注意取余与除法各自的作用，不要搞反了
            carry = sum / 10;
        }

        return sb.reverse().toString();
    }
}

References

415. Add Strings