Problem p03745 (C++)

Buggy submission (status: Runtime Error):
/* ---------- STL Libraries ---------- */

// IO library
#include <cstdio>
#include <iomanip>
#include <ios>
#include <iostream>

// algorithm library
#include <algorithm>
#include <cmath>
#include <numeric>

// container library
#include <deque>
#include <list>
#include <map>
#include <queue>
#include <set>
#include <string>
#include <vector>

/* ---------- Namespace ---------- */

using namespace std;

/* ---------- Type Abbreviation ---------- */

template <typename T> using V = vector<T>;
template <typename T, typename U> using P = pair<T, U>;

using ll = long long;
using PL = P<ll, ll>;
using PS = P<string, ll>;

using VI = V<int>;
using VVI = V<VI>;
using VL = V<ll>;
using VVL = V<VL>;
using VS = V<string>;
using VB = V<bool>;
using VVB = V<VB>;
using VPL = V<PL>;
using VPS = V<PS>;

#define fst first
#define snd second

/* ---------- conversion ---------- */

#define INT(c) static_cast<int>(c)
#define CHAR(n) static_cast<char>(n)
#define LL(n) static_cast<ll>(n)
#define DOUBLE(n) static_cast<double>(n)

/* ---------- container ---------- */

#define EACH(i, c) for (auto i = (c).begin(); i != (c).end(); i++)
#define ALL(c) (c).begin(), (c).end()
#define SORT(c) sort(ALL(c))
#define GSORT(c) sort(ALL(c), greater<decltype((c).front())>())
#define SZ(x) (LL((x).size()))

/* ---------- repetition ---------- */

#define FOR(i, a, b) for (ll i = (a); i <= (b); i++)
#define REP(i, n) FOR(i, 0, n - 1)
#define NREP(i, n) FOR(i, 1, n)

#define RFOR(i, a, b) for (ll i = (a); i >= (b); i--)
#define RREP(i, n) RFOR(i, n, 0)
#define RNREP(i, n) RFOR(i, n, 1)

// Usual REP runs from 0 to n-1 (R: n to 0)
// Natural REP runs from 1 to n (R: n to 1)

/* ---------- Constants ---------- */

// const ll MOD = 1e9 + 7;
// const int INF = 1 << 25;
// const ll INF = 1LL << 50;
// const double PI = 3.14159265358979;
// const ll dx[4] = {0, -1, 1, 0};
// const ll dy[4] = {-1, 0, 0, 1};
// const ll dx[8] = {-1, 0, 1, -1, 1, -1, 0, 1};
// const ll dy[8] = {-1, -1, -1, 0, 0, 1, 1, 1};

/* ---------- Short Functions ---------- */

template <typename T> T square(T a) { return a * a; }

/* ---------- Debug ---------- */
#define GET_VAR_NAME(variable) #variable

#define test(x) cout << GET_VAR_NAME(x) << " = " << x << endl;
#define testP(p)                                                               \
  cout << GET_VAR_NAME(p) << " = (" << (p).fst << ", " << (p).snd << ")"       \
       << endl;

/* ^^^^^^^^^^^^^^^ Settting Part ^^^^^^^^^^^^^^^ */

/* ---------- Variances ---------- */

/* ---------- Functions ---------- */

/* ---------- Main Function ---------- */

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);

  int N;
  cin >> N;

  VI change(N - 1);
  int preA;
  cin >> preA;

  REP(i, N - 1) {
    int A;
    cin >> A;
    if (A > preA) {
      change[i] = 1;
    } else if (A < preA) {
      change[i] = -1;
    } else {
      change[i] = 0;
    }
    preA = A;
  }

  int iter = 0;
  int ans = 1;

  while (true) {
    int state = change[iter];
    iter++;
    while (iter <= N - 2) {
      if (abs(change[iter] - state) > 1) {
        ans++;
        iter++;
        break;
      } else {
        if (state == 0) {
          state = change[iter];
        }
        iter++;
      }
    }
    if (iter >= N - 2) {
      cout << ans << endl;
      return 0;
    }
  }
  cout << "error" << endl;
  return 0;
}

Fixed submission:
/* ---------- STL Libraries ---------- */

// IO library
#include <cstdio>
#include <iomanip>
#include <ios>
#include <iostream>

// algorithm library
#include <algorithm>
#include <cmath>
#include <numeric>

// container library
#include <deque>
#include <list>
#include <map>
#include <queue>
#include <set>
#include <string>
#include <vector>

/* ---------- Namespace ---------- */

using namespace std;

/* ---------- Type Abbreviation ---------- */

template <typename T> using V = vector<T>;
template <typename T, typename U> using P = pair<T, U>;

using ll = long long;
using PL = P<ll, ll>;
using PS = P<string, ll>;

using VI = V<int>;
using VVI = V<VI>;
using VL = V<ll>;
using VVL = V<VL>;
using VS = V<string>;
using VB = V<bool>;
using VVB = V<VB>;
using VPL = V<PL>;
using VPS = V<PS>;

#define fst first
#define snd second

/* ---------- conversion ---------- */

#define INT(c) static_cast<int>(c)
#define CHAR(n) static_cast<char>(n)
#define LL(n) static_cast<ll>(n)
#define DOUBLE(n) static_cast<double>(n)

/* ---------- container ---------- */

#define EACH(i, c) for (auto i = (c).begin(); i != (c).end(); i++)
#define ALL(c) (c).begin(), (c).end()
#define SORT(c) sort(ALL(c))
#define GSORT(c) sort(ALL(c), greater<decltype((c).front())>())
#define SZ(x) (LL((x).size()))

/* ---------- repetition ---------- */

#define FOR(i, a, b) for (ll i = (a); i <= (b); i++)
#define REP(i, n) FOR(i, 0, n - 1)
#define NREP(i, n) FOR(i, 1, n)

#define RFOR(i, a, b) for (ll i = (a); i >= (b); i--)
#define RREP(i, n) RFOR(i, n, 0)
#define RNREP(i, n) RFOR(i, n, 1)

// Usual REP runs from 0 to n-1 (R: n to 0)
// Natural REP runs from 1 to n (R: n to 1)

/* ---------- Constants ---------- */

// const ll MOD = 1e9 + 7;
// const int INF = 1 << 25;
// const ll INF = 1LL << 50;
// const double PI = 3.14159265358979;
// const ll dx[4] = {0, -1, 1, 0};
// const ll dy[4] = {-1, 0, 0, 1};
// const ll dx[8] = {-1, 0, 1, -1, 1, -1, 0, 1};
// const ll dy[8] = {-1, -1, -1, 0, 0, 1, 1, 1};

/* ---------- Short Functions ---------- */

template <typename T> T square(T a) { return a * a; }

/* ---------- Debug ---------- */
#define GET_VAR_NAME(variable) #variable

#define test(x) cout << GET_VAR_NAME(x) << " = " << x << endl;
#define testP(p)                                                               \
  cout << GET_VAR_NAME(p) << " = (" << (p).fst << ", " << (p).snd << ")"       \
       << endl;

/* ^^^^^^^^^^^^^^^ Settting Part ^^^^^^^^^^^^^^^ */

/* ---------- Variances ---------- */

/* ---------- Functions ---------- */

/* ---------- Main Function ---------- */

int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);

  int N;
  cin >> N;

  VI change(N + 1, 0);
  int preA;
  cin >> preA;

  REP(i, N - 1) {
    int A;
    cin >> A;
    if (A > preA) {
      change[i] = 1;
    } else if (A < preA) {
      change[i] = -1;
    } else {
      change[i] = 0;
    }
    preA = A;
  }

  int iter = 0;
  int ans = 1;

  while (true) {
    int state = change[iter];
    iter++;
    while (iter <= N - 2) {
      if (abs(change[iter] - state) > 1) {
        ans++;
        iter++;
        break;
      } else {
        if (state == 0) {
          state = change[iter];
        }
        iter++;
      }
    }
    if (iter >= N - 2) {
      cout << ans << endl;
      return 0;
    }
  }
  cout << "error" << endl;
  return 0;
}