BigIntegerAddition.java

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

 Notice

The length of both num1 and num2 is < 5100.
Both num1 and num2 contains only digits 0-9.
Both num1 and num2 does not contain any leading zero.
You must not use any built-in BigInteger library or convert the inputs to
integer directly.
Example
Given num1 = "123", num2 = "45"
return "168"
 */
public class BigIntegerAddition {

    /*
     * @param num1: a non-negative integers
     * @param num2: a non-negative integers
     * @return: return sum of num1 and num2
     */
    public String addStrings(String num1, String num2) {
        String result = "";
        int carrier = 0;
        for (int i = num1.length() - 1, j = num2.length() - 1;
                i >= 0 || j >= 0;) {
            int sum = carrier;
            if (i >= 0) {
                sum += num1.charAt(i--) - '0';
            }
            if (j >= 0) {
                sum += num2.charAt(j--) - '0';
            }
            result = (sum % 10) + result;
            carrier = sum / 10;

        }
        result = carrier > 0 ? (carrier + result) : result;
        return result;
    }

}