求sum=1+111+1111+........+1....111 .

1，思路

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;
}
}