Use subtraction

[cyq7on]

class Solution {
    public char findTheDifference(String s, String t) {
        int sub = 0 ;
        for (int i = 0; i < s.length(); i++) {
            sub += t.charAt(i) -  s.charAt(i) ;
        }
        int c = t.charAt(t.length() - 1) - '0';
        sub += c;
        return (char) (sub + '0');
    }
}