如下是一份没有写完的高精度代码。

• Q1：BASE 和 WIDTH 起什么作用？
• Q2：按这样的格式，高精四则的剩下三个如何写？

struct BigInteger {
	static const int BASE = 100000000;
	static const int WIDTH = 8;
	vector<int> s;
	BigInteger(long long num = 0) {*this = num;}
	BigInteger operator = (long long num) {
		s.clear();
		do {
			s.push_back(num % BASE);
			num /= BASE;
		} while(num > 0);
		return *this;
	}
	BigInteger operator = (const string& str) {
		s.clear();
		int x, len = (str.length() - 1) / WIDTH + 1;
		for(int i = 0; i < len; i++) {
			int end = str.length() - i * WIDTH;
			int start = max(0, end - WIDTH);
			sscanf(str.substr(start, end - start).c_str(), "%d", &x);
			s.push_back(x);
		}
		return *this;
	}
	ostream& operator << (ostream &out, const BigInteger& x) {
		out << x.s.back();
		for(int i = x.s.size() - 2; i >= 0; i--) {
			char buf[20];
			sprintf(buf, "%08d", x.s[i]);
			for(int j = 0; j < strlen(buf); j++)
				out << buf[j];
		}
		return out;
	}
	istream& operator >> (istream &in, BigInteger& x) {
		string s;
		if(!(in >> s)) return in;
		x = s;
		return in;
	}
	BigInteger operator + (const BigInteger& b) const {
		BigInteger c;
		c.s.clear();
		for(int i = 0, g = 0; ; i++) {
			if(g == 0 && i >= s.size() && i >= b.s.size()) break;
			int x = g;
			if(i < s.size()) x += s[i];
			if(i < b.s.size()) x += b.s[i];
			c.s.push_back(x % BASE);
			g = x / BASE;
		}
		return c;
	}
	BigInteger operator += (const BigInteger& b) {
		*this = *this + b;
		return *this;
	}
};