POJ.2121 Inglish-Number Translator

1.Problem
http://poj.org/problem?id=2121

2.Idea
Calculate digits for three parts of million, thousand, unit.

3.Source

 string words[] =  
 { "zero", "one", "two", "three",  
 "four", "five", "six", "seven",  
 "eight","nine", "ten", "eleven", "twelve",  
 "thirteen", "fourteen", "fifteen", "sixteen",  
 "seventeen", "eighteen", "nineteen", "twenty",  
 "thirty", "forty", "fifty", "sixty", "seventy",  
 "eighty", "ninety","hundred", "thousand", "million" };  
 int num2str[] = { 0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,30,40,50,60,70,80,90,100,1000,1000000 };  
 int main()  
 {  
      map<string, int> mp;  
      for (int i = 0; i < 31; i++) {  
           mp[words[i]] = num2str[i];  
      }  
      string str, line;  
      while (getline(cin, line)) {  
           if (line.length() == 0) break;  
           istringstream input(line);  
           bool neg = 0;  
           ll ans = 0, pre = 0;  
           while (input >> str) {  
                if (str == "negative") neg = 1;  
                else if (str == "hundred") pre *= 100;  
                else if (str == "thousand") {  
                     pre *= 1000;  
                     ans += pre;  
                     pre = 0;  
                }  
                else if (str == "million") {  
                     pre *= 1000000;  
                     ans += pre;  
                     pre = 0;  
                }  
                else {  
                     pre += mp[str];  
                }  
           }  
           ans += pre;  
           if (neg) ans *= -1;  
           cout << ans << endl;  
      }  
      return 0;  
 }