AddTwoLargeNumbers.java

import java.util.*;
public class AddTwoLargeNumbers {
    static String findSum(String str1, String str2)
    {
        if (str1.length() > str2.length()){
            String t = str1;
            str1 = str2;
            str2 = t;
        }

        String str = "";

        int n1 = str1.length(), n2 = str2.length();

        str1 = new StringBuilder(str1).reverse().toString();
        str2 = new StringBuilder(str2).reverse().toString();

        int carry = 0;
        for (int i = 0; i < n1; i++)
        {
            int sum = ((int)(str1.charAt(i) - '0') +
                    (int)(str2.charAt(i) - '0') + carry);
            str += (char)(sum % 10 + '0');
            carry = sum / 10;
        }

        for (int i = n1; i < n2; i++)
        {
            int sum = ((int)(str2.charAt(i) - '0') + carry);
            str += (char)(sum % 10 + '0');
            carry = sum / 10;
        }

        if (carry > 0)
            str += (char)(carry + '0');

        str = new StringBuilder(str).reverse().toString();

        return str;
    }

    public static void main(String[] args)
    {
        String str1 = "198212156835";
        String str2 = "198111523228";

        System.out.println("Sum of " + str1 + " and " + str2 + " = " + findSum(str1, str2));        // the ans should be = 396323680063
    }
}