702. 连接两个字符串中的不同字符

string (1)     //可以直接查找字符串对象，
size_t find (const string& str, size_t pos = 0) const noexcept;
c-string (2)    //从类型的字符串
size_t find (const char* s, size_t pos = 0) const;
buffer (3)       //从pos开始查找s的前n个字符
size_t find (const char* s, size_t pos, size_type n) const;
character (4)    //查找字符
size_t find (char c, size_t pos = 0) const noexcept;

string concatenetedString(string &s1, string &s2) {
        string res;
        if(s2.size()==0)
        return s1;
        if(s1.size()==0)
        return s2;
        for(int i=0;i<s1.size();i++)
        {
            if(s2.find(s1[i])==string::npos)
            res+=s1[i];
        }
        for(int i=0;i<s2.size();i++)
        {
            if(s1.find(s2[i])==string::npos)
            res+=s2[i];
        }
        return res;
        // write your code here
    }