Difference between s=s+c and s+=c?

101

I was trying this question: https://leetcode.com/problems/sort-characters-by-frequency/submissions/

How is s = s + c different from s+=c? where s is a string and c is a character.

I kept getting a memory limit exceeded error. Here's my code:

class Solution {
public:
    struct comp {
        bool operator()(const pair<char,int> a,const pair<char,int> b) {
            return a.second <b.second;
        }
    };
    string frequencySort(string s) {
        priority_queue<pair<char,int>,vector<pair<char,int> >,comp> pq;
        unordered_map<char,int> umap;
        for(int i=0;i<s.length();i++)
            umap[s[i]]++;
        for(unordered_map<char,int>::iterator it=umap.begin();it!=umap.end();it++) {
            pq.push(make_pair(it->first,it->second));
        }
        string res = "";
        s="";
        while(!pq.empty()) {
            pair<char,int> temp = pq.top();
            for(int i=0;i<temp.second;i++)
                res = res + temp.first;  // Error
            pq.pop();
        }
        return res;
    }
};

Apparently, I finally found the error to be at the line marked in the code. I changed it to res+=temp.first and it worked! Any ideas why an assignment statement like this can cause a memory limit exceeded error?

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