高精度加法

class Solution {
    public String addStrings(String num1, String num2) {
        String ret ="";
        int i = num1.length() - 1, j = num2.length() - 1, flag = 0;
        while(i >= 0 || j >= 0){
            int num = flag;
            if(i >= 0 && j >= 0) num = num + (num1.charAt(i) - '0') + (num2.charAt(j) - '0');
            else if(i >= 0) num = num + (num1.charAt(i) - '0');
            else num = num + (num2.charAt(j) - '0');

            ret = (num % 10) + ret;
            flag = num / 10;
            
            i--;
            j--;
        }
        if(flag == 1) ret = "1" + ret;
        return ret;
    }
}