C++ string类，字符拼接方法优劣

 string类push_back()方法比用+号空间耗费少，在力扣剑指 Offer 05. 替换空格题目中下面两种方法分别消耗空间6MB，11MB

class Solution {
public:
    string replaceSpace(string s) {
        string s1="";
        for(char e:s){
            if(e==' '){
                s1.push_back('%');
                s1.push_back('2');
                s1.push_back('0');
            }
            else s1.push_back(e);
        }
        return s1;
    }
};

class Solution {
public:
    string replaceSpace(string s) {
        string s1="";
        for(char e:s){
            if(e==' '){
                s1=s1+"%20";
            }
            else s1=s1+e;
        }
        return s1;
    }
};