Problem p03202 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <algorithm>
#include <cmath>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#define _USE_MATH_DEFINES
#include <functional>
#include <math.h>
using namespace std;

#define rep(i, x) for (ll i = 0; i < x; i++)
#define repn(i, x) for (ll i = 1; i <= x; i++)

typedef long long ll;
const ll INF = 1e17;
const ll MOD = 1000000007;
const ll MAX = 4000001;

ll max(ll a, ll b) {
  if (a > b) {
    return a;
  }
  return b;
}

ll min(ll a, ll b) {
  if (a > b) {
    return b;
  }
  return a;
}

ll gcd(ll a, ll b) {
  if (b == 0) {
    return a;
  }
  if (a < b) {
    return gcd(b, a);
  }
  return gcd(b, a % b);
}

ll lcm(ll a, ll b) { return a / gcd(a, b) * b; }

struct edge {
  ll ind;
  ll fr;
  ll to;
  ll d;
};

class mint {
  long long x;

public:
  mint(long long x = 0) : x((x % MOD + MOD) % MOD) {}
  mint operator-() const { return mint(-x); }
  mint &operator+=(const mint &a) {
    if ((x += a.x) >= MOD)
      x -= MOD;
    return *this;
  }
  mint &operator-=(const mint &a) {
    if ((x += MOD - a.x) >= MOD)
      x -= MOD;
    return *this;
  }
  mint &operator*=(const mint &a) {
    (x *= a.x) %= MOD;
    return *this;
  }
  mint operator+(const mint &a) const {
    mint res(*this);
    return res += a;
  }
  mint operator-(const mint &a) const {
    mint res(*this);
    return res -= a;
  }
  mint operator*(const mint &a) const {
    mint res(*this);
    return res *= a;
  }

  mint operator^(const ll &t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }

  mint pow(ll t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }
  // for prime MOD
  mint inv() const { return pow(MOD - 2); }
  mint &operator/=(const mint &a) { return (*this) *= a.inv(); }
  mint operator/(const mint &a) const {
    mint res(*this);
    return res /= a;
  }

  friend ostream &operator<<(ostream &os, const mint &m) {
    os << m.x;
    return os;
  }
};

//////////////////////////

ll N;
vector<ll> A;

bool ch(ll X) {
  stack<pair<ll, ll>> st;

  for (ll i = 2; i <= N; i++) {
    if (A[i] > A[i - 1]) {
      continue;
    }

    while (!st.empty() && st.top().first > A[i]) {
      st.pop();
    }

    if (!st.empty() && st.top().first == A[i]) {
      auto p = st.top();
      st.pop();
      st.push({p.first, p.second + 1});
    }

    else {
      st.push({A[i], 1});
    }

    while (st.top().second == X) {
      auto p = st.top();
      st.pop();
      if (!st.empty() && st.top().first == p.first - 1) {
        auto q = st.top();
        st.pop();
        st.push({q.first, q.second + 1});
      } else {
        st.push({p.first - 1, 1});
      }

      if (st.top().first == 0) {
        return false;
      }
    }
  }

  return true;
}

int main() {
  cin >> N;
  A.assign(N + 1, 0);
  repn(i, N) cin >> A[i];

  ll le = 0;
  ll ri = MAX;

  while (ri - le > 1) {
    ll m = (ri + le) / 2;
    if (ch(m)) {
      ri = m;
    } else {
      le = m;
    }
  }

  cout << ri;

  system("PAUSE");
}

Fixed submission:
#include <algorithm>
#include <cmath>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <stack>
#include <string>
#define _USE_MATH_DEFINES
#include <functional>
#include <math.h>
using namespace std;

#define rep(i, x) for (ll i = 0; i < x; i++)
#define repn(i, x) for (ll i = 1; i <= x; i++)

typedef long long ll;
const ll INF = 1e17;
const ll MOD = 1000000007;
const ll MAX = 4000001;

ll max(ll a, ll b) {
  if (a > b) {
    return a;
  }
  return b;
}

ll min(ll a, ll b) {
  if (a > b) {
    return b;
  }
  return a;
}

ll gcd(ll a, ll b) {
  if (b == 0) {
    return a;
  }
  if (a < b) {
    return gcd(b, a);
  }
  return gcd(b, a % b);
}

ll lcm(ll a, ll b) { return a / gcd(a, b) * b; }

struct edge {
  ll ind;
  ll fr;
  ll to;
  ll d;
};

class mint {
  long long x;

public:
  mint(long long x = 0) : x((x % MOD + MOD) % MOD) {}
  mint operator-() const { return mint(-x); }
  mint &operator+=(const mint &a) {
    if ((x += a.x) >= MOD)
      x -= MOD;
    return *this;
  }
  mint &operator-=(const mint &a) {
    if ((x += MOD - a.x) >= MOD)
      x -= MOD;
    return *this;
  }
  mint &operator*=(const mint &a) {
    (x *= a.x) %= MOD;
    return *this;
  }
  mint operator+(const mint &a) const {
    mint res(*this);
    return res += a;
  }
  mint operator-(const mint &a) const {
    mint res(*this);
    return res -= a;
  }
  mint operator*(const mint &a) const {
    mint res(*this);
    return res *= a;
  }

  mint operator^(const ll &t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }

  mint pow(ll t) const {
    if (!t)
      return 1;
    mint a = pow(t >> 1);
    a *= a;
    if (t & 1)
      a *= *this;
    return a;
  }
  // for prime MOD
  mint inv() const { return pow(MOD - 2); }
  mint &operator/=(const mint &a) { return (*this) *= a.inv(); }
  mint operator/(const mint &a) const {
    mint res(*this);
    return res /= a;
  }

  friend ostream &operator<<(ostream &os, const mint &m) {
    os << m.x;
    return os;
  }
};

//////////////////////////

ll N;
vector<ll> A;

bool ch(ll X) {
  stack<pair<ll, ll>> st;

  for (ll i = 2; i <= N; i++) {
    if (A[i] > A[i - 1]) {
      continue;
    }

    if (X == 1) {
      return false;
    }

    while (!st.empty() && st.top().first > A[i]) {
      st.pop();
    }

    if (!st.empty() && st.top().first == A[i]) {
      auto p = st.top();
      st.pop();
      st.push({p.first, p.second + 1});
    }

    else {
      st.push({A[i], 1});
    }

    while (st.top().second == X) {
      auto p = st.top();
      st.pop();
      if (!st.empty() && st.top().first == p.first - 1) {
        auto q = st.top();
        st.pop();
        st.push({q.first, q.second + 1});
      } else {
        st.push({p.first - 1, 1});
      }

      if (st.top().first == 0) {
        return false;
      }
    }
  }

  return true;
}

int main() {
  cin >> N;
  A.assign(N + 1, 0);
  repn(i, N) cin >> A[i];

  ll le = 0;
  ll ri = MAX;

  while (ri - le > 1) {
    ll m = (ri + le) / 2;
    if (ch(m)) {
      ri = m;
    } else {
      le = m;
    }
  }

  cout << ri;

  system("PAUSE");
}