// 自写
#include <iostream>

using namespace std;

float HarmonicMean(float num1, float num2);

int main()
{
	float num1;
	float num2;
	do
	{
		cout << "请输入第一个数字：";
		cin >> num1;
		cout << "请输入第二个数字：";
		cin >> num2;
		if (num1 == 0 || num2 == 0)
		{
			break;
		}
		cout << num1 << "和" << num2 << "调和平均数是：" << HarmonicMean(num1, num2) << endl;
	} while (num1 != 0 || num2 != 0);

	return 0;
}

float HarmonicMean(float num1, float num2)
{
	return (2 * num1*num2) / (num1+num2);
}

Teacher：

#include <iostream>

using namespace std;

float harmonicMean(float a, float b);

int main()
{
	float num1, num2;
	cout << "请输入两个数的值：" << endl;
	while (cin >> num1 >> num2 && num1 != 0 && num2 != 0)
	{
		cout << num1 << "和" << num2 << "的调和平均数为：";
		cout << harmonicMean(num1, num2) << endl;
		cout << "请输入两个数的值：" << endl;
	}
	return 0;
}

float harmonicMean(float a, float b)
{
	return 2 * a * b / (a + b);
}