Language: C++

#include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <exception>
#include <fstream>
#include <functional>
#include <iomanip>
#include <ios>
#include <iosfwd>
#include <iostream>
#include <istream>
#include <iterator>
#include <limits>
#include <list>
#include <locale>
#include <map>
#include <memory>
#include <new>
#include <numeric>
#include <ostream>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <stdexcept>
#include <streambuf>
#include <string>
#include <typeinfo>
#include <utility>
#include <valarray>
#include <vector>
#include <climits>
#include <cstring>

#define rep(i, m, n) for(int i=int(m);i<int(n);i++)
#define all(c) begin(c),end(c)

template<typename T1, typename T2>
inline void chmin(T1 &a, T2 b) { if (a > b) a = b; }

template<typename T1, typename T2>
inline void chmax(T1 &a, T2 b) { if (a < b) a = b; }

//改造
typedef long long int ll;
using namespace std;
#define INF (1 << 30) - 1
#define INFl (ll)5e15
#define DEBUG 0 //デバッグする時1にしてね
#define dump(x)  cerr << #x << " = " << (x) << endl
#define MOD 1000000007


//ここから編集する
class UnionFind {
    vector<int> p;//p[i]はiの属する組織
public:
    UnionFind(int n) {
        p = vector<int>(n);
        for (int i = 0; i < n; i++) {
            p[i] = i;
        }
        return;
    }

/* xの属する集合を返す */
    int find(int x) {
        if (p[x] == x) return x;
        return p[x] = find(p[x]);
    }

/* yにxを統合する */
    void unite(int x, int y) {
        p[find(x)] = p[find(y)];
    }

/* xとyが属する集合が同じかを判定する */
    bool same(int x, int y) {
        return find(x) == find(y);
    }

};

class Solve {
public:
    int N;
    ll D;
    vector<ll> A;

    void input() {
        cin >> N >> D;
        A.resize(N);
        rep(i, 0, N) cin >> A[i];
    }

    struct edge {
        int a;
        int b;
        ll w;
    };
    vector<edge> edges;

    void left() {
        //右から見ていけ
        //どこを参照すべきかを教えてくれる。
//        vector<int> ret(N, -1);
        int key = N - 1;
        ll val = A[N - 1];
        for (int i = N - 2; i >= 0; --i) {
            val += D;
//            ret[i] = key;
            edges.push_back(edge{key, i, val + A[i]});
            if (A[i] <= val) {
                val = A[i];
                key = i;
            }
        }
    }

    void right() {
        int key = 0;
        ll val = A[0];
        for (int i = 1; i < N; ++i) {
            val += D;
//            edges.emplace_back(key, i, val + A[i]);
            edges.push_back(edge{key, i, val + A[i]});
            if (A[i] <= val) {
                val = A[i];
                key = i;
            }

        }
    }

    void solve() {
        input();
        left();
        right();
        UnionFind uf(N);

        sort(all(edges), [](edge a, edge b) {
            return a.w < b.w;
        });

        ll ans = 0ll;
        int cnt = 0;
        for (auto e : edges) {
            if (!uf.same(e.a, e.b)) {
                uf.unite(e.a, e.b);
                ans += e.w;
                cnt++;
            }
        }

        assert(cnt == N - 1);

        cout << ans << endl;


    }
};


int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(10);

    Solve().solve();


    return 0;
}


Compiler output:
a.cc: In member function 'void Solve::solve()':
a.cc:156:9: error: 'assert' was not declared in this scope
  156 |         assert(cnt == N - 1);
      |         ^~~~~~
a.cc:35:1: note: 'assert' is defined in header '<cassert>'; this is probably fixable by adding '#include <cassert>'
   34 | #include <cstring>
  +++ |+#include <cassert>
   35 |