set应用：提取文本中所有不同的单词

set是一个常用的容器，是数学意义上的集合–每个元素最多只能出现一次。和sort函数一样，自定义的类型(struct、class)也可以构造set，但是同样必须定义“小于”运算符。
例题：
输入一个文本，找出所有不同的单词，按照字典序从小到大输出。单词不区分大小写，大写的单词按小写输出。这里的单词定义为连续的字母序列。
样例输入：

I am a student from HUST.

样例输出

a
am
from
hust
i
student

程序代码:

#include<iostream>
#include<string>
#include<set>
#include<sstream>
using namespace std;
set<string> dict;//string 集合
int main()
{
    string s,buf;
    while(cin>>s)
    {
        for(int i=0;i<s.length();i++)
        {
            if(isalpha(s[i]))
                s[i]=tolower(s[i]);
            else 
                s[i]=' ';
        }
            stringstream ss(s);
            while(ss>>buf)
                dict.insert(buf);

    }
    for(set<string>::iterator it = dict.begin();it!=dict.end();it++)
        cout<<*it<<endl;
    return 0;
}

程序运行结果：

输入文本：

Adventures in Disneyland

Two blondes were going to Disneyland. 

They were driving on the Interstate when they saw a sign that said Disneyland LEFT. 

They started crying and turned around and went home.

运行结果：