Sum = 1 + 111 + 1111 +... + 1... 111.

1. Ideas

If you use Int or long to add a large number, you must use string for storage.

2. Code

public class LargeNumAdd{    public static void main(String[] args)    {        String sum = "0";        for (int i = 1; i <= 2009; i++)        {            String temp = getStr(i);            sum = getSum(sum, temp);        }        System.out.println("sum: " + sum);    }    public static String getSum(String a, String b)    {        if (a.length() > b.length())        {            for (int i = b.length(); i < a.length(); i++)            {                b = ‘0‘ + b;            }        } else        {            for (int i = a.length(); i < b.length(); i++)            {                a = ‘0‘ + a;            }        }        char array1[] = a.toCharArray();        char array2[] = b.toCharArray();        String sum = "";        int carrayIn = 0;        for (int i = a.length() - 1; i >= 0; i--)        {            sum = getRemain(array1[i], array2[i], carrayIn) + sum;            carrayIn = getCarry(array1[i], array2[i], carrayIn);        }        return sum;    }        public static String getRemain(char a, char b, int carrayIn)    {        int sum = (a - ‘0‘) + (b - ‘0‘) + carrayIn;        String rem = sum % 10 + "";        return rem;    }    private static int getCarry(char a, char b, int carryIn)    {        int sum = (a - ‘0‘) + (b - ‘0‘) + carryIn;        int carry = sum / 10;        return carry;    }    public static String getStr(int N)    {        String s = "";        for (int i = 0; i < N; i++)        {            s += "1";        }        return s;    }}

 

Sum = 1 + 111 + 1111 +... + 1... 111.