<p>Given a string <code>paragraph</code> and a string array of the banned words <code>banned</code>, return <em>the most frequent word that is not banned</em>. It is <strong>guaranteed</strong> there is <strong>at least one word</strong> that is not banned, and that the answer is <strong>unique</strong>.</p>

<p>The words in <code>paragraph</code> are <strong>case-insensitive</strong> and the answer should be returned in <strong>lowercase</strong>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> paragraph = &quot;Bob hit a ball, the hit BALL flew far after it was hit.&quot;, banned = [&quot;hit&quot;]
<strong>Output:</strong> &quot;ball&quot;
<strong>Explanation:</strong> 
&quot;hit&quot; occurs 3 times, but it is a banned word.
&quot;ball&quot; 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 &quot;ball,&quot;), 
and that &quot;hit&quot; isn&#39;t the answer even though it occurs more because it is banned.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> paragraph = &quot;a.&quot;, banned = []
<strong>Output:</strong> &quot;a&quot;
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= paragraph.length &lt;= 1000</code></li>
	<li>paragraph consists of English letters, space <code>&#39; &#39;</code>, or one of the symbols: <code>&quot;!?&#39;,;.&quot;</code>.</li>
	<li><code>0 &lt;= banned.length &lt;= 100</code></li>
	<li><code>1 &lt;= banned[i].length &lt;= 10</code></li>
	<li><code>banned[i]</code> consists of only lowercase English letters.</li>
</ul>


class Solution {
public:
    unordered_set<char> non_letter;
    void split_str(string s, vector<string>& res){
        if(s.size() ==0) return ;
        while(non_letter.count(s[0]))
            s.erase(0, 1);
        if(s.size() ==0) return;
        while(non_letter.count(s[s.size() -1]))
            s.erase(s.size() -1);
        int i = 0;
        while(i < s.size()){
            int j = i;
            while(j < s.size() && !non_letter.count(s[j])){
                j++;
            }
            string data = s.substr(i, j -i);
            std::transform(data.begin(), data.end(), data.begin(),
                [](unsigned char c){ return std::tolower(c); });
            res.push_back(data);

            while(j < s.size() && non_letter.count(s[j]))
                ++j;
            i = j;
        }

    }
    string mostCommonWord(string paragraph, vector<string>& banned) {
        unordered_set<string> ban_strs(banned.begin(), banned.end());
        string no_letters = "!?',;. ";
        non_letter = unordered_set<char>(no_letters.begin(), no_letters.end());
        vector<string> strs;
        split_str(paragraph, strs);
        unordered_map<string, int> m;
        for(string s : strs){
            if(!ban_strs.count(s))
                m[s]++;
        }
        string ans = "";
        int maxi = -1;
        for(auto& [k, f] : m){
            if(f > maxi){
                maxi = f;
                ans = k;
            }
        }
        return ans;
    }
};
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