大数计算求(3^n+1)/2

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

const int MAX_LEN = 10000;

int main()
{
	int n = 0;

	//3^n的最大位数是：10000,乘一次3就加一位
	while (cin >> n) {
		//初始状态
		vector<int> res(MAX_LEN, 0);
		res[0] = 1;

		//大数乘法
		while (n--) {
			for (int i = 0; i < MAX_LEN - 1; ++i) {
				res[i] *= 3;
			}
			for (int i = 0; i < MAX_LEN - 1; ++i) {
				res[i + 1] += res[i] / 10;
				res[i] %= 10;
			}
		}
		
		++res[0]; // 3^n + 1
		for (int i = 0; i < MAX_LEN - 1; ++i) {
			res[i + 1] += res[i] / 10;
			res[i] %= 10;
		}

		//大数除2,从高位开始
		for (int i = MAX_LEN - 1; i > 0; --i) {
			res[i - 1] += 10 * (res[i] % 2);
			res[i] /= 2;
		}
		res[0] /= 2;

		bool bMeetNoZero = false;
		for (int i = MAX_LEN - 1; i >= 0; --i) {
			if (bMeetNoZero) {
				cout << res[i];
			} else {
				if (res[i] != 0) {
					cout << res[i];
					bMeetNoZero = true;
				}
			}
		}
		cout << endl;
	}

	return 0;
}