将判断字母数字和变为小写用相应的库函数

#include <iostream>
#include <unordered_map>
#include <cctype>
using namespace std;

int main()
{
    string str;
    getline(cin, str);

    unordered_map<string, int> hash;

    for (int i = 0; i < str.size(); i ++ )
        if (isalnum(str[i]))
        {
            string word;
            int j = i;
            while (j < str.size() && isalnum(str[j])) word += tolower(str[j ++ ]);

            hash[word] ++ ;
            i = j;
        }

    string word;
    int cnt = -1;
    for (auto item : hash)
        if (item.second > cnt || (item.second == cnt && item.first < word))
        {
            word = item.first;
            cnt = item.second;
        }

    cout << word << ' ' << cnt << endl;

    return 0;
}