力扣415 字符串相加

public String addStrings(String num1, String num2) {
        StringBuilder res=new StringBuilder();
        int i=num1.length()-1;
        int j=num2.length()-1;
        int carry=0;
        while(i>=0||j>=0){
            int n1=i>=0?num1.charAt(i)-'0':0;
            int n2=j>=0?num2.charAt(j)-'0':0;
            int temp=n1+n2+carry;
            carry=temp/10;
            res.append(temp%10);
            i--;
            j--;
        }
        if(carry==1) res.append('1');
        return res.reverse().toString();
    }