6以前WA 6以后TLE 6AC

代码如下

#include <iostream>
#include <cmath>
#include <string>
#include <algorithm>
using namespace std;
string str(int n){
    string s;
    int i = 0;
    while(n){
        s[i] = n % 10;
        ++i;
        n /= 10;
    }
    return s;
}
bool prime(int n){
    if(n == 1 || n == 0) return false;
    if(n == 2) return true;
    for(int i = 2; i <= sqrt(n); ++i) if(n % i == 0) return false;
    return true;
}
int main(){
    ios::sync_with_stdio(0);
    cin.tie(0);
    int a, b;
    cin >> a >> b;
    for(int i = a; i <= b; ++i){
        string t1 = str(i), t2 = t1;
        reverse(t1.begin(), t1.end());
        if(prime(i) == true && t1 == t2)
            cout << i << endl;
    }
    return 0;
}

求求您帮帮这个$juruo$吧