我相信目前没有比我更简单的解法

# include <bits/stdc++.h>
using namespace std;
bool is(int n)
{
    string s = to_string(n);
    string a = s;
    reverse(a.begin() , a.end());
    return a == s;
}
int main()
{
    int n;
    cin >> n;
    for(int i = 1 ; i <= n ; i++)
        if(is(i))
            cout << i << endl;
}