反转单词

【思路】：每个单词反转，然后整体反转

class Solution {
public:
    string ReverseSentence(string str) {
        if(!str.size())return "";
        int pre = 0;
        for(int i = 0;i < str.size();++i){
            if(str[i] == ' '){
                reverse(str.begin()+pre, str.begin()+i);
                while(str[i] == ' ')++i;
                pre = i;
            }
        }
        reverse(str.begin()+pre, str.end());
        reverse(str.begin(), str.end());
        return str;
    }
};