C++大数相乘

Given two numbers represented as strings, return multiplication of the numbers as a string.

string multiply(string num1, string num2)
{
	string ans = "";
	int n1 = num1.size();
	int n2 = num2.size();
	if (n1 == 0 || n2 == 0) return ans;
	vector<int> results(n1 + n2);
	for (int i = n2-1,ii=0; i >= 0; --i,++ii){
		int en = 0;
		for (int j = n1-1,jj=0; j >= 0; --j,++jj){
			int tmp = results[ii + jj] + (num1[j] - '0')*(num2[i] - '0') + en;
			results[ii + jj] = tmp % 10;
			en = tmp / 10;
		}
		results[ii + n1] = en;
	}
	int idx = 0;
	for (int i = n1 + n2 - 1; i >= 0; --i){
		if (results[i]>0){
			idx = i;
			break;
		}
	}
	for (int i = idx; i >= 0; --i){
		ans += (results[i] + '0');
	}
	return ans;
}


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