Molar Mass, ACM/ICPC Seoul 2007,UVa1586

AC且在VS2019无警告代码：

#include <iostream>
#include <string>
#include <cctype>
#include <iomanip>
using namespace std;
double atom(char);
int main()
{
	int t;
	cin >> t;
	while (t-- > 0)
	{
		string s;
		cin >> s;
		double sum = 0, len = s.length();
		char pre = 0;//存储上一个元素
		for (int i = 0; i < len; ++i)
		{
			if (isalpha(s[i]))
			{
				sum += atom(s[i]);
				pre = s[i];
			}
			else if (isdigit(s[i]))
			{
				int num = s[i] - '0';
				while (isdigit(s[i + 1]))
				{
					num = num * 10 + s[++i] - '0';
				}
				sum += (static_cast<double>(num) - 1) * atom(pre);//-1是因为上一个元素已加进了总和中
			}
		}
		cout << fixed << setprecision(3);
		cout << sum << endl;
	}
}
double atom(char ch)
{
	if (ch == 'C')
		return 12.01;
	if (ch == 'H')
		return 1.008;
	if (ch == 'O')
		return 16.00;
	if (ch == 'N')
		return 14.01;
	return 0;
}