Project Euler：Problem 48 Self powers

The series, 1¹ + 2² + 3³ + ... + 10¹⁰ = 10405071317.

Find the last ten digits of the series, 1¹ + 2² + 3³ + ... + 1000¹⁰⁰⁰.

#include <iostream>
#include <string>
using namespace std;

string powe(int n)
{
	string s = "1";
	for (int i = 1; i <= n; i++)
	{
		int len = s.length();
		int flag = 0;
		for (int j = len - 1; j >= 0; j--)
		{
			int tp = (s[j] - '0') * n + flag;
			flag = tp / 10;
			s[j] = tp % 10 + '0';
		}
		if (flag != 0)
		{
			while (flag)
			{
				char a = flag % 10 + '0';
				s = a + s;
				flag /= 10;
			}
		}
	}
	return s;
}

string pl(string a, string b)   //加法
{
	string res = "";
	if (a.length() < b.length())
	{
		string t = a;
		a = b;
		b = t;
	}
	int lo = a.length();
	int sh = b.length();
	string s(lo - sh, '0');
	b = s + b;
	int flag = 0;
	for (int i = lo - 1; i >= 0; i--)
	{
		int tmp = a[i] + b[i] - '0' - '0' + flag;
		int low = tmp % 10;
		flag = tmp / 10;
		char a1 = low + '0';
		res = a1 + res;
	}

	if (flag != 0)
		res = "1" + res;
	return res;
}

int main()
{
	string res = "0";
	for (int i = 1; i <= 1000; i++)
	{
		string tmp = powe(i);
		res = pl(res, tmp);
	}
	int len = res.length();

	string ans = "";
	for (int i = len - 1 - 9; i < len; i++)
		ans = ans + res[i];
	cout << ans << endl;
	system("pause");
	return 0;
}