public class Solution {
    public String addStrings(String num1, String num2) {
        int len1=num1.length()-1;
        int len2=num2.length()-1;
        StringBuilder sb=new StringBuilder();
        int sum=0,carry=0;
        while(len1>=0||len2>=0){
            int a=len1>=0?num1.charAt(len1)-‘0‘:0;
            int b=len2>=0?num2.charAt(len2)-‘0‘:0;
            sum=a+b+carry;
            if(sum>9){
                sb.insert(0,sum%10);
                carry=1;
                sum=0;
            }else{
                sb.insert(0,sum);
                sum=0;
                carry=0;
            }
            len1--;
            len2--;
        }
        if(carry==1){
            sb.insert(0,1);
        }
        return sb.toString();
    }
}