The description of problem

Given a string paragraph and a string array of the banned words banned, return the most frequent word that is not banned. It is guaranteed there is at least one word that is not banned, and that the answer is unique.

The words in paragraph are case-insensitive and the answer should be returned in lowercase.

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/most-common-word

Chinese version

给定一个段落 (paragraph) 和一个禁用单词列表 (banned)。返回出现次数最多，同时不在禁用列表中的单词。

题目保证至少有一个词不在禁用列表中，而且答案唯一。

禁用列表中的单词用小写字母表示，不含标点符号。段落中的单词不区分大小写。答案都是小写字母。

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/most-common-word
著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

example

Input: paragraph = "Bob hit a ball, the hit BALL flew far after it was hit.", banned = ["hit"]
Output: "ball"
Explanation: 
"hit" occurs 3 times, but it is a banned word.
"ball" occurs twice (and no other word does), so it is the most frequent non-banned word in the paragraph. 
Note that words in the paragraph are not case sensitive,
that punctuation is ignored (even if adjacent to words, such as "ball,"), 
and that "hit" isn't the answer even though it occurs more because it is banned.

My codes

 #include <string>
#include <iostream>
#include <vector>
#include <sstream>
#include <algorithm>
#include <iterator>
#include <unordered_map>

using namespace std;
class Solution {
public:
     // define toLowerCase()
    string toLowerCase(string str) {
        stringstream ss;
        for(int i = 0; i < str.size(); i++) {
            if(str[i] >= 'A' && str[i] <= 'Z') {
                ss << (char)(str[i] + 32);
            } else {
                ss << str[i];
            }
        }
        return ss.str();
    }
    //convert a sentence to a list of words and dissociate punctuations and spaces
    vector<string> split(string str) {
        vector<string> res;
        stringstream ss;
        for(int i = 0; i < str.size(); i++) {
            if(str[i] == ' ' || str[i] == ',' || str[i] == '.' || str[i] == ';' || str[i] == ':' || str[i] == '?' || str[i] == '!' || str[i] == '\'') {
                if(ss.str() != "") {
                    res.push_back(ss.str());
                    ss.str("");
                }
            } else {
                ss << str[i];
            }
        }
        if(ss.str() != "") {
            res.push_back(ss.str());
        }
        return res;
    }
    // sort unordered_map<string, int> m from the largest to the smallest by value and return key corresponding to the largest value
    string getMax(unordered_map<string, int> m) {
        string res;
        int max = 0;
        for (auto it = m.begin(); it != m.end(); it++) {
            if (it->second > max) {
                max = it->second;
                res = it->first;
            }
        }
        return res;
    }
    string mostCommonWord(string paragraph, vector<string>& banned) {
        vector<string> words = split(paragraph);
        unordered_map<string, int> m;
        for (auto word : words) {
            string w = toLowerCase(word);
            if (m.find(w) == m.end()) {
                m[w] = 1;
            } else {
                m[w]++;
            }
            if (find(banned.begin(), banned.end(), w) != banned.end()) {
                m.erase(w);
            }
        }
        // output the elements in the map
        for (auto it = m.begin(); it != m.end(); it++) {
            cout << it->first << " " << it->second << endl;
        }
        return getMax(m);
    }
};

int main()
{
    Solution s;
    string paragraph = "j. t? T. z! R, v, F' x! L; l! W. M; S. y? r! n; O. q; I? h; w. t; y; X? y, p. k! k, h, J, r? w! U! V; j' u; R! z. s. T' k. P? M' I' j! y. P, T! e; X. w? M! Y, X; G; d, X? S' F, K? V, r' v, v, D, w, K! S? Q! N. n. V. v. t? t' x! u. j; m; n! F, V' Y! h; c! V, v, X' X' t? n; N' r; x. W' P? W; p' q, S' X, J; R. x; z; z! G, U; m. P; o. P! Y; I, I' l' J? h; Q; s? U, q, x. J, T! o. z, N, L; u, w! u, S. Y! V; S? y' E! O; p' X, w. p' M, h! R; t? K? Y' z? T? w; u. q' R, q, T. R? I. R! t, X, s? u; z. u, Y, n' U; m; p? g' P? y' v, o? K? R. Q? I! c, X, x. r' u! m' y. t. W; x! K? B. v; m, k; k' x; Z! U! p. U? Q, t, u' E' n? S' w. y; W, x? r. p! Y? q, Y. t, Z' V, S. q; W. Z, z? x! k, I. n; x? z; V? s! g, U; E' m! Z? y' x? V! t, F. Z? Y' S! z, Y' T? x?";
    vector<string> banned = {"m","q","e","l","c","i","z","j","g","t","w","v","h","p","d","b","a","r","x","n"};
    cout << s.mostCommonWord(paragraph, banned) << endl;

    return 0;
}