415_Add Strings

/*
415. Add Strings
Runtime: 9 ms, faster than 16.45% of Java online submissions for Add Strings.
Memory Usage: 39.9 MB, less than 46.12% of Java online submissions for Add Strings.*/

class Solution {
    public String addStrings(String num1, String num2) {
         String fs ="";
          int i=num1.length()-1 , j=num2.length()-1,sum=0;
        
        while(i>=0 || j>=0 || sum>=1)
        {
            sum+=((i>=0)?num1.charAt(i)-'0': 0);
            sum+=((j>=0)?num2.charAt(j)-'0': 0);
            fs=(char)(sum%10+'0')+fs;
            sum/=10;
            
            i--;j--;
            
        }
        
        return fs;
    
    }
}