Given two non-negative integers num1 and num2 represented as string, return the sum of num1 and num2.

Note:

1. The length of both num1 and num2 is < 5100.
2. Both num1 and num2 contains only digits 0-9.
3. Both num1 and num2 does not contain any leading zero.
4. You must not use any built-in BigInteger library or convert the inputs to integer directly.

There are 2 things to note in this problem:

1. Use StringBuilder to concatenate string is much more efficient (Do need to reverse it for the right order)
2. No need to separate the for-loop, just check if the pointer exceed boundary, if so, just set number = 0

class Solution {
    public String addStrings(String num1, String num2) {
        char[] c1 = num1.toCharArray();    
        char[] c2 = num2.toCharArray();
        int n1=0, n2=0, carry=0;
        StringBuilder res = new StringBuilder();

        for(int i=c1.length-1,j=c2.length-1;i>-1||j>-1||carry>0;i--,j--){
            n1=(i>-1)?c1[i]-'0':0;
            n2=(j>-1)?c2[j]-'0':0;
            int sum = n1+n2+carry;
            if(sum>9) {
                carry=1;
                res.append(sum%10);
            }else{
                carry=0;
                res.append(sum);
            }
        }

        return res.reverse().toString();

    }
}