415 Add Strings

Problem:

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.

Solutions:

public class Solution {
    public String addStrings(String num1, String num2) {
        num1 = new StringBuilder(num1).reverse().toString();
        num2 = new StringBuilder(num2).reverse().toString();
        String result = "";
        int carry = 0;
        for (int i = 0; i < num1.length() || i < num2.length(); i ++) {
            int c1 = 0;
            int c2 = 0;
            if (i < num1.length()) {
                c1 = num1.charAt(i) - '0';
            }
            if (i < num2.length()) {
                c2 = num2.charAt(i) - '0';
            }
            int sum = c1 + c2 + carry;
            int digit = sum % 10;
            carry = sum / 10;
            result = digit + result;
        }
        if (carry != 0) {
            result = carry + result;
        }
        return result;
    }
}