LeetCode Most Common Word 最多见的词

Given a paragraph and a list of banned words, return the most frequent word that is not in the list of banned words. It is guaranteed there is at least one word that isn't banned, and that the answer is unique.
Words in the list of banned words are given in lowercase, and free of punctuation. Words in the paragraph are not case sensitive. The answer is in lowercase.

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.

Note:

• 1 <= paragraph.length <= 1000.
• 1 <= banned.length <= 100.
• 1 <= banned[i].length <= 10.
• The answer is unique, and written in lowercase (even if its occurrences in paragraph may have uppercase symbols, and even if it is a proper noun.)
• paragraph only consists of letters, spaces, or the punctuation symbols !?',;.
• Different words in paragraph are always separated by a space.
• There are no hyphens or hyphenated words.
• Words only consist of letters, never apostrophes or other punctuation symbols.

这是一道字符串处理的题目，给咱们一个字符串段落，以及一个被禁止的单词，段落中咱们要处理!?',;.这些标点符号。咱们能够算出各个单词出现的次数，而后将被禁止的单词排除在外，就能够算出出现次数最多的单词的哪个。
解法一：

class Solution {
public:
    string mostCommonWord(string paragraph, vector<string>& banned) {
        map<string,int> M;
        map<string,int>::iterator it;
        M.clear();
        int i,j,n=paragraph.size(),m=banned.size();
        string s;
        for(i=0;i<paragraph.length();i++)if(paragraph[i]>='A'&&paragraph[i]<='Z')paragraph[i]+='a'-'A';
        for(i=0;i<n;i=j)
        {
            s="";
            for(j=i;j<n&&(paragraph[j]<'a'||paragraph[j]>'z');j++);
            if(j==n)break;
            for(;j<n&&paragraph[j]>='a'&&paragraph[j]<='z';j++)s+=paragraph[j];
            M[s]++;
        }
        for(i=0;i<m;i++)M[banned[i]]=0;
        for(it=M.begin(),i=0;it!=M.end();it++)if(it->second>i)
        {
            s=it->first;
            i=it->second;
        }
        return s;
    }
};