AddStrings.java

public class Solution {
    public String addStrings(String num1, String num2) {
        StringBuilder sb = new StringBuilder();
        int carry = 0;
        for(int i=num1.length()-1,j=num2.length()-1; i>=0||j>=0||carry==1;i--,j--)
        {
            int a = i<0? 0 : num1.charAt(i) - '0';
            int b = j<0? 0 : num2.charAt(j)-'0';
            int sum = a+b+carry;
            sb.append(sum%10);
            carry = sum/10;
        }
        return sb.reverse().toString();
    }
}