Строки Фибоначчи

#include <bits/stdc++.h>
 
using namespace std;
 
int main()
{
    string s1, s2, str;
    int K, P;
    cin >> K >> P;
    s1 = "a";
    s2 = "bc";
    for (int i = 1; i <= 100000000; i++)
    {
        if (i == 1)
        {
            str = s1;
        }
        else if (i == 2)
        {
            str = s2;
        }
        else
        {
            str = s1 + s2;
            s1 = s2;
            s2 = str;
        }
        if (i == K) 
        {
        if (isalpha(str[P-1])) cout << str[P-1] << endl;
        else cout << "No solution" << endl;
        break;
        }
    }
    system("pause");
    return 0;
}

#include <iostream>
using namespace std;
 
int fs[] = { 0,1,1,2,3,5,8,13,21,34,55,89,144,233,377,610,987,1597,2584,4181,6765,10946,17711,28657,46368,75025,121393,196418,317811,514229,832040,1346269,2178309,3524578,5702887,9227465,14930352,24157817,39088169,63245986,102334155 };
int m = sizeof(fs) / sizeof(fs[0]);
 
char f(int k, int p) {
    int d = k<=m ? fs[k-1] : fs[m-1];
    return  k==1 ? (p==1 ? 'a' : '?') :
            k==2 ? (p==1 ? 'b' : p==2 ? 'c' : '?') :
            p<=d ? f(k-2, p) :
            f(k-1, p-d);
}
int main() { int k, p; cin >> k >> p; char c = f(k, p);
            if (c=='?') cout << "No solution"; else cout << c; }

#include <iostream>
#include <string>
#include <vector>
#include <cassert>
 
std::string a = "a";
std::string bc = "bc";
 
int main()
{
    const int str_number = 20;
    const int char_pos = 46;
 
    std::vector<std::string> str_fib{a, bc, a + bc};
    for (size_t i = 3; i < str_number; ++i)
    {
        //size_t size_to_check = (str_fib.end() - 2)->size() + (str_fib.end() - 1)->size();
        str_fib.push_back(*(str_fib.end() - 2) + *(str_fib.end() - 1));
        //assert(str_fib.back().size() == size_to_check);
    }
 
    //for (size_t i = 0; i < str_fib.size(); ++i)
        //std::cout << "[" << i << "]: " << str_fib[i] << "\n";
 
    std::cout << "Res: ";
    try
    {
        std::cout << str_fib.at(str_number - 1).at(char_pos - 1) << std::endl;
    }
    catch (...)
    {
        std::cout << "No solution\n";
    }
}