关于string"+"的复杂度

#include<bits/stdc++.h>
using namespace std;
const int maxn = 600005;
#define endl '\n'
#define art pair<int,int>
#define fir first
#define sec second
string s="$";

 ....
  
int main()
{
    ios::sync_with_stdio(0);
    cin.tie(0),cout.tie(0);
    int n;
    cin>>n;
    int ans26,ansB;
    long long sum=0;
    int kmp=0;
    for(int i=1;i<=n;i++)
    {
        char ch;
        cin>>ch;
        cin>>w[i];
        if(i!=1)
        {
            ch=(ch-'a'+ans)%26+'a';
            w[i]^=ansB;
        }else
        {
            ans26=w[1];
            ansB=w[1];
        }
        s+=ch;    
  		....
    }
    return 0;
}