求sum=1+111+1111+........+1....111 .

1，思路

大数相加，若直接使用int，或者long都会超出长度，因此考虑使用String存储。

2，代码

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;
    }
}