Language: C++

 #include <iostream>
#include <cstdio>
#include <algorithm>
#include <cmath>
#include <vector>
#include <list>
#include <set>
#include <map>
#include <queue>
#include <stack>
#include <cctype>
#include <cassert>
#include <climits>
#include <string>
#include <bitset>
#include <cfloat>
#include <unordered_set>
#include <iomanip>
using namespace std;
typedef long double ld;
typedef long long int ll;
typedef unsigned long long int ull;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;
typedef vector<int> vi;
typedef vector<char> vc;
typedef vector<bool> vb;
typedef vector<double> vd;
typedef vector<string> vs;
typedef vector<ll> vll;
typedef vector<pair<int,int> > vpii;
typedef vector<vector<int> > vvi;
typedef vector<vector<char> > vvc;
typedef vector<vector<string> > vvs;
typedef vector<vector<ll> > vvll;
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define rrep(i,n) for(int i = 1; i <= (n); ++i)
#define irep(it, stl) for(auto it = stl.begin(); it != stl.end(); it++)
#define drep(i,n) for(int i = (n) - 1; i >= 0; --i)
#define fin(ans) cout << (ans) << endl
#define mp(p,q) make_pair(p, q)
#define pb(n) push_back(n)
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define Sort(a) sort(a.begin(), a.end())
#define Rort(a) sort(a.rbegin(), a.rend())
#define MATHPI acos(-1)
int dx8[8] = {1, 0, -1, 0, 1, -1, -1, 1};
int dy8[8] = {0, 1, 0, -1, 1, 1, -1, -1};
int dx4[4] = {1, 0, -1, 0};
int dy4[4] = {0, 1, 0, -1};
template <class T> inline bool chmax(T& a,T b){if(a<b){a=b;return 1;} return 0;}
template <class T> inline bool chmin(T& a,T b){if(a>b){a=b;return 1;} return 0;}
template<typename A, size_t N, typename T> void Fill(A (&array)[N], const T &val){ fill((T*)array, (T*)(array+N), val); }
struct io{io(){ios::sync_with_stdio(false);cin.tie(0);}};
const int INF = INT_MAX;
const ll LLINF = 1LL<<60;
const ll MOD = 1000000007;
const double EPS = 1e-9;
template < typename T > inline string toString( const T &a ) { ostringstream oss; oss << a; return oss.str(); };

template<class T>
vector<T> make_vec(size_t a){
    return vector<T>(a);
}
template<class T, class... Ts>
auto make_vec(size_t a, Ts... ts){
  return vector<decltype(make_vec<T>(ts...))>(a, make_vec<T>(ts...));
}

// mod add
const int MOD = 1000000007;
void add(long long &a, long long b) {
    a += b;
    if (a >= MOD) a -= MOD;
}

//0-なんでも可、1-A,2-G,3-C,4-T
//AGC,ACG,GAC,AGXC,AXGC、つまり123,132,213,1203,1023を含まない
long long solve(int N) {
    auto dp = make_vec<long long>(N+1, 5, 5, 5); // dp[i文字][3文字前][2文字前][1文字前]
    dp[0][0][0][0] = 1;
    for (int n = 0; n < N; ++n) {
        for (int i = 0; i < 5; ++i) {
            for (int j = 0; j < 5; ++j) {
                 for (int k = 0; k < 5; ++k) {
                     for (int l = 1; l < 5; ++l) {
                         if (i == 1 && j == 2 && l == 3) continue;
                         if (i == 1 && k == 2 && l == 3) continue;
                         if (j == 1 && k == 2 && l == 3) continue;
                         if (j == 2 && k == 1 && l == 3) continue;
                         if (j == 1 && k == 3 && l == 2) continue;
                          //次の3文字はj,k,lの順
                         add(dp[n+1][j][k][l], dp[n][i][j][k]);
                     }
                }
            }
        }
    }
    long long res = 0;
    for (int i = 1; i < 5; ++i)
        for (int j = 1; j < 5; ++j)
            for (int k = 1; k < 5; ++k)
                add(res, dp[N][i][j][k]);
    return res;
}

int main() {
    int N;
    while (cin >> N) cout << solve(N) << endl;
}



Compiler output:
a.cc:72:11: error: conflicting declaration 'const int MOD'
   72 | const int MOD = 1000000007;
      |           ^~~
a.cc:58:10: note: previous declaration as 'const ll MOD'
   58 | const ll MOD = 1000000007;
      |          ^~~