Leetcode sword finger offer jz73 flip word sequence

  This question is mainly about string Class object string understanding and use :

The code comments are as follows

#include<iostream>
using namespace std;
class Solution {
public:
    string ReverseSentence(string str) {
        if (str.empty()) return str;  // In special cases, it directly returns a null value 
       
        string ret = "";   // Set two empty strings 
        string tmp = "";
        for (int i = str.size()- 1; i >= 0; i--) {   // Because the string is inverted , Therefore, traverse forward from the tail 
            //  Merge a word 
            if (str[i] != ' ') {   
                tmp = str[i]+tmp;   // Note the order in which strings are added   New characters should be in tmp Before character  
            }
            //  Find a word , Merge words into the result string 
            else if (str[i] == ' ' ) {
                ret = ret+tmp + " ";  // ditto   New words should be in   Before the original word , The order cannot be changed 
                tmp = "";

            }
        }
       
        return ret+tmp;   // Once you jump out of the loop, the length of the returned string is   In both cases, the sum of strings . And note that the order cannot be changed   New words should be in front of the string 
 
    }