problemas-para-competicao-pibiti-arthur-dolival/decode-ways/src/alternative_ac.cpp

#include <bits/stdc++.h>

typedef long long ll;
using namespace std;

const int MOD = 1e9 + 7;

ll numDecodings(string s)
{
    if (s[0] == '0')
        return 0;

    int N = s.size();
    ll dp1 = 1, dp2 = 1;
    for (int i = 2; i <= N; i++)
    {
        ll next = 0;
        char current = s[i - 1], last = s[i - 2];
        string aux = "";
        aux += last;
        aux += current;
        if (current != '0')
        {
            next = dp2;
            next %= MOD;
        }

        int code = stoi(aux);
        if (10 <= code && code <= 26)
        {
            next += dp1;
            next %= MOD;
        }
        swap(dp1, dp2);
        swap(dp2, next);
    }

    return dp2;
}

int main()
{
    int n;
    cin >> n;
    string code;
    cin >> code;
    cout << numDecodings(code) << endl;
    return 0;
}