Uva1586 Molar Mass，ACM/ICPC Seoul 2007

发现从后往前扫描字符串会更方便。

#include <stdio.h>
#include <ctype.h>
#include <string.h>

const double Hydrogen = 1.008;
const double Oxygen     = 16.00;
const double Carbon = 12.01;
const double Nitrogen = 14.01;

double p(int);

int main()
{
    int t;
    char molecule[85];
    scanf("%d", &t);
    getchar();
    while(t--)
    {
        double mass = 0;
        double tm;
        scanf("%s", molecule);
        int len = strlen(molecule);
        int digit = 0;
        double num = 0;
        while(len--)
        {
            if(isdigit(molecule[len]))
            {
                ++digit;
                num += (int)(molecule[len] - '0') * p(digit);
                continue;
            }
            else if('C' == molecule[len]) tm = Carbon;
            else if('H' == molecule[len]) tm = Hydrogen;
            else if('O' == molecule[len]) tm = Oxygen;
            else if('N' == molecule[len]) tm = Nitrogen;

            if(!digit) num = 1;
            mass += tm * num;
            digit = 0;
            num = 0;
        }
        printf("%.3lf\n", mass);;
    }
    return 0;
}

double p(int digit)
{
    int i;
    double sum = 1;
    for(i = 1; i < digit; ++i)
        sum *= 10;
    return sum;
}