LeetCode--Reverse Words in a String

class Solution {
public:
    void reverseWords(string &s) {
        int len = s.length();
        if(len == 0)
            return;
        vector<string> res;
        int i = 0;
        int j = len-1;
        //去掉收尾的空格
        while(i < len && s[i] == ' ')
            ++i;
        while(j >= 0 && s[j] == ' ')
            --j;
        int k = i;//寻找一个单词
        while(i <= j)
        {
            k = i;
            while(i<=j && s[i]!=' ')
            {
                ++i;
            }
            res.push_back(s.substr(k,i-k));
            while(s[i]==' ' && i<=j)
                ++i;
        }
        if(res.empty())
        {
            s = "";
            return;
        }
        reverse(res.begin(),res.end());
        string m = res[0];
        for(int i=1;i<res.size();i++){  
            m+=" ";  
            m+=res[i];  
        }  
        s=m; 
    }
};

先删除前后的空格，多个空格合并为一个空格输出。