set中元素已经从小到大排好序

stringstream常见用法介绍 https://blog.csdn.net/liitdar/article/details/82598039

isalpha:判断一个字符是否是字母 tolower:把给定的字母转换为小写字母

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