Problem p02953 (C++)

Buggy submission (status: Runtime Error):
#include <bits/stdc++.h>
using namespace std;

// 総数を1000000007で割った余り
const long long mod = 1e9 + 7;

using ll = long long;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
#define ull unsigned long long
#define ld long double
#define vi vector<int>
#define vll vector<ll>
#define vc vector<char>
#define vs vector<string>
#define vpii vector<pii>
#define vpll vector<pll>

#define rep(i, n) for (int i = 0, i##_len = (n); i < i##_len; i++)
#define rep1(i, n) for (int i = 1, i##_len = (n); i <= i##_len; i++)
#define repr(i, n) for (int i = ((int)(n)-1); i >= 0; i--)
#define rep1r(i, n) for (int i = ((int)(n)); i >= 1; i--)

#define sz(x) ((int)(x).size())
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()

#define SORT(v, n) sort(v, v + n);
// #define SORT(v, n) stable_sort(v, v + n);
#define VSORT(v) sort(v.begin(), v.end());
#define RSORT(x) sort(rall(x));
#define pb push_back
#define mp make_pair

#define INF (1e9)
#define PI (acos(-1))
#define EPS (1e-7)

ull gcd(ull a, ull b) { return b ? gcd(b, a % b) : a; }
ull lcm(ull a, ull b) { return a / gcd(a, b) * b; }

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  // cout << fixed << setprecision(5);
  ll n;
  cin >> n;
  vector<ll> h(n + 1);
  h[n + 1] = INF;
  h[0] = 0;
  rep1(i, n) { cin >> h[i]; }

  bool flg = false;
  for (int i = n; i >= 1; i--) {
    if (h[i - 1] - h[i] > 1) {
      flg = false;
      break;
    }
    if (h[i - 1] - h[i] == 1) {
      h[i - 1]--;
    }
    flg = true;
  }

  if (flg) {
    cout << "Yes" << endl;
  } else {
    cout << "No" << endl;
  }

  return 0;
}


Fixed submission:
#include <bits/stdc++.h>
using namespace std;

// 総数を1000000007で割った余り
const long long mod = 1e9 + 7;

using ll = long long;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
#define ull unsigned long long
#define ld long double
#define vi vector<int>
#define vll vector<ll>
#define vc vector<char>
#define vs vector<string>
#define vpii vector<pii>
#define vpll vector<pll>

#define rep(i, n) for (int i = 0, i##_len = (n); i < i##_len; i++)
#define rep1(i, n) for (int i = 1, i##_len = (n); i <= i##_len; i++)
#define repr(i, n) for (int i = ((int)(n)-1); i >= 0; i--)
#define rep1r(i, n) for (int i = ((int)(n)); i >= 1; i--)

#define sz(x) ((int)(x).size())
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()

#define SORT(v, n) sort(v, v + n);
// #define SORT(v, n) stable_sort(v, v + n);
#define VSORT(v) sort(v.begin(), v.end());
#define RSORT(x) sort(rall(x));
#define pb push_back
#define mp make_pair

#define INF (1e9)
#define PI (acos(-1))
#define EPS (1e-7)

ull gcd(ull a, ull b) { return b ? gcd(b, a % b) : a; }
ull lcm(ull a, ull b) { return a / gcd(a, b) * b; }

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  // cout << fixed << setprecision(5);
  ll n;
  cin >> n;
  vector<ll> h(n + 2);
  h[n + 1] = INF;
  h[0] = 0;
  rep1(i, n) { cin >> h[i]; }

  bool flg = false;
  for (int i = n; i >= 1; i--) {
    if (h[i - 1] - h[i] > 1) {
      flg = false;
      break;
    }
    if (h[i - 1] - h[i] == 1) {
      h[i - 1]--;
    }
    flg = true;
  }

  if (flg) {
    cout << "Yes" << endl;
  } else {
    cout << "No" << endl;
  }

  return 0;
}