Recommended reading Left-handed string The principles are based on:

I. solution I

class Solution {
public:
    string ReverseSentence(string str) {
       int len = str.size();
        if (len == 0) return "";
        string res = str;
        for(int i = len - 1; i >= 0; --i){
            if(str[i] == ' ')
                res[len - 1 - i] = str[i];
            else{
                int begin = i, end = i;
                while (begin >= 1 && str[begin - 1] != ' ') begin--;
                for(int k = begin; k <= end; ++k,--i)
                    res[len - 1 - i] = str[k];
                i += 1;
            }
        }
        return res;
    }
};

II. Solution II

class Solution {
public:
    string ReverseSentence(string str) {
        int len = str.size();
        if (len == 0) return "";
        for(int i = 0; i < len; ++i)
            if(str[i] != ' '){
                int begin = i,end = i;
                while(end < len && str[end] != ' ')end++; 
                end == len - 1?i = end: i = --end;
                myReverse(str, begin, end);
            }
        myReverse(str, 0, len - 1);
        return str;
    }
     
    void myReverse(string &str, int begin, int end){    //Internal exchange function
        int len = str.size();    //String length
        if(begin < 0 || end >= len)    //begin and end are just Subscripts
            return;
        while(begin < end){
            char temp = str[begin];
            str[begin] = str[end];
            str[end] = temp;
            begin++;end--;
        }
    }  
};

3, Solution 3

Traverse from the front to the back. When you encounter a space, press the previously read one to the front of the result and add a space. Finally, the loop ends; if the string used to read the string is not empty, press it to the front of the result again, of course, this time there is no need to add a space at the front of the result.

class Solution {
public:
    string ReverseSentence(string str) {
        string res = "", tmp = "";
        for(int i = 0; i < str.size(); ++i){
            if(str[i] == ' ') {
                res = " " + tmp + res;//After a space is encountered, press the previously read result to the front and add a space
                tmp = "";
            }
            else 
                tmp += str[i];
        }
        if(tmp.size()) res = tmp + res;
        return res;
    }
};