Tìm Fibo thứ n

#include <bits/stdc++.h>

using namespace std;

void fillZero(string &a, string &b)
{
    int lengthMax = max(a.length(), b.length());
    while (a.length() < lengthMax) a = '0' + a;
    while (b.length() < lengthMax) b = '0' + b;
}

int SoSanh(string a, string b)
{
    if (a.length() > b.length()) return 1;
    else if (a.length() < b.length()) return -1;

    for (int i = 0; i < a.length(); i++)
    {
        int x = a[i] - 48;
        int y = b[i] - 48;
        if (x > y) return 1;
        else if (x < y )return -1;
    }
    return 0;
}

string Cong(string a, string b)
{
    int du = 0;
    string res = "";
    fillZero(a, b);
    for (int i = a.length() - 1; i >= 0; i--)
    {
        int x = a[i] -48;
        int y = b[i] -48;
        int temp = x + y + du;
        if ( i == 0)
        {
            stringstream ss;
            string s;
            ss << temp;
            ss >> s;
            res = s + res;
            break;
        }
        res = (char)(temp % 10 + 48) + res;
        du = temp / 10;
    }
    while (res.length() > 1 && res[0] == '0') res.erase(0, 1);
    return res;
}


string genFibo(int n)
{
    string f1 = "1", f2 = "1", f3 = "2";
    if (n == 1 || n == 2 ) return f1;
    for (int i = 3; i <= n; i++)
    {
        f3 = Cong(f1, f2);
        f1 = f2;
        f2 = f3;
    }
    return f3;
}


int main()
{
    cin.tie(0);
    ios_base::sync_with_stdio(0);
    int n;
    cin >> n;
    cout << genFibo(n);
    return 0;
}