LeetCode Most Common Word 最多見的詞

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.

這是一道字符串處理的題目，給咱們一個字符串段落，以及一個被禁止的單詞，段落中咱們要處理!?',;.這些標點符號。咱們能夠算出各個單詞出現的次數，而後將被禁止的單詞排除在外，就能夠算出出現次數最多的單詞的哪個。
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;
    }
};