上流程圖：

有了流程圖，寫代碼就可以一個分支一個分支地寫了：

#define NDEBUG//這個宏的意思是不要debug，交上去的時候要有這一句。//#define LOCAL//定義了這個宏，就是輸出到本地，否則是輸出到屏幕#include <stdio.h>#include <iomanip>#include <iostream>#include <cmath>#include <ctime>#include <cassert>#include <string>#include <vector>const double C = 12.01, H = 1.008, O = 16.00, N = 14.01;using namespace std;int main(int argc, char* argv[]){#ifdef LOCAL	FILE* newFile;	freopen_s(&newFile, "out.txt", "w", stdout);#endif	cout << setiosflags(ios_base::fixed) << setPRecision(3);	int n; cin >> n;	while (n--)	{		string Word; cin >> word;		double sum = 0;		for (int i = 0; i < word.size(); ++i)		{			int howmany = 0;			if (i + 1 == word.size() || isalpha(word[i + 1]))			{				howmany = 1;				switch (word[i])				{				case 'C':					sum += C * howmany;					break;				case 'H':					sum += H * howmany;					break;					//氧原子				case 'O':					sum += O * howmany;					break;				case 'N':					sum += N * howmany;					break;				}				if (i + 1 == word.size())				{					cout << sum << endl;				}				continue;			}			//得記住讀取數(shù)字之前的i值啊……			int j = i;			while (i < word.size() && isdigit(word[i + 1]))			{				howmany *= 10;				howmany += word[i + 1] - '0';				++i;			}			switch (word[j])			{			case 'C':				sum += C * howmany;				break;			case 'H':				sum += H * howmany;				break;				//氧原子			case 'O':				sum += O * howmany;				break;			case 'N':				sum += N * howmany;				break;			}			if (i + 1 == word.size())			{				cout << sum << endl;			}			continue;		}	}	return 0;}