Is there difference in string concatenation using shortand operator?

Question

I tried appending character to end of the string using + operator to a coding problem .That solution was giving memory limit exceeded. Then I saw solutions using += to append character. Is there any difference between the two in case of time complexity or memory complexity ?

Example - My solution

string arrangeWords(string text) {
 text[0] = text[0] + 32;
 text = text + ' ';
 string temp = "";
 map < int, vector < string >> mp;
 for (char c: text) {
  if (c != ' ')
   temp = temp + c; //---Notice this  line
  else {
   mp[temp.size()].push_back(temp);
   temp = "";
  }
 }
 string res = "";
 for (auto it: mp)
  for (auto j: it.second)
   res = res + j + ' '; //----Notice this line

 res[0] = toupper(res[0]);
 return res.substr(0, res.size() - 1);
}

Accepted Solution -

string arrangeWords(string text) {
 text[0] += 32;
 text += ' ';
 string temp = "";
 map < int, vector < string >> mp;
 for (char c: text) {
  if (c != ' ')
   temp += c; //Notice this line change
  else {
   mp[temp.size()].push_back(temp);
   temp = "";
  }
 }
 string res = "";
 for (auto it: mp)
  for (auto j: it.second)
   res += j + ' '; //Notice this line change

 res[0] = toupper(res[0]);
 return res.substr(0, res.size() - 1);
}

Answer 1

temp = temp + c; creates a temporary temp + c (which would probably require (slow) allocation) before to assign to temp. (Complexity is O(n))

temp += c; reuse buffer if large enough (complexity O(1)), else reallocation should be done (and then, it would be "similar" to above method) (complexity O(n)). (amortized complexity is O(1))

In addition, less allocation occurs, less chance to have fragmented memory.