Решение задачи Последовательность(2) с Меньшиков

#include <iostream>
#include <cstdio>
#include <vector>
 
using namespace std;
 
const int size = 1010;
long long n;
vector<int> mas;
vector<int> mem;
vector<int> pos;
void input()
{
    mas.resize(size);
    mem.resize(size);
    pos.resize(size);
    cin>>mas[0]>>mas[1]>>mas[2];
    mem[mas[2] + 10*mas[1] + 100*mas[0]] = 1;
 
    cin>>n;
    n--;
}
void solve()
{
    int base = -1;
    int len = -1;
    for (int i=3;i<size;i++)
    {
        mas[i] = (mas[i-1] + mas[i-2] + mas[i-3])%10;
        int value = mas[i-1] + 10*mas[i-2] + 100*mas[i-3];
        if (mem[value] && i!=3)
        {
            base = pos[value];
            len = i - 3 - base;
            break;
        }
        mem[value] = 1;
        pos[value] = i-3;
    }
    if (n < base + len)
        cout<<mas[n];
    else
    {
        int index = base + (n - base) % len;
        cout<<mas[index];
    }
}
int main()
{
    freopen("input.txt","r",stdin);
    freopen("output.txt","w",stdout);
 
    input();
    solve();
    return 0;
}