Problem p02824 (C++)

Buggy submission (status: Time Limit Exceeded):
#include <bits/stdc++.h>
#define lsb(x) (x & (-x))
#define ll long long
#define ull unsigned long long
#define uint unsigned int

#if 0
const int MOD = (int) ;

inline int lgput(int a, int b) {
    int ans = 1;
    while(b > 0) {
        if(b & 1) ans = (1LL * ans * a) % MOD;
        b >>= 1;
        a = (1LL * a * a) % MOD;
    }
    return ans;
}

inline void mod(int &x) {
    if(x >= MOD)
        x -= MOD;
}

inline void add(int &x, int y) {
    x += y;
    mod(x);
}

inline void sub(int &x, int y) {
    x += MOD - y;
    mod(x);
}

inline void mul(int &x, int y) {
    x = (1LL * x * y) % MOD;
}

inline int inv(int x) {
    return lgput(x, MOD - 2);
}
#endif

#if 0
int fact[], invfact[];

inline void prec(int n) {
    fact[0] = 1;
    for(int i = 1; i <= n; i++) {
        fact[i] = (1LL * fact[i - 1] * i) % MOD;
    }
    invfact[n] = lgput(fact[n], MOD - 2);
    for(int i = n - 1; i >= 0; i--) {
        invfact[i] = (1LL * invfact[i + 1] * (i + 1)) % MOD;
    }
}

inline int comb(int n, int k) {
    if(n < k) return 0;
    return (1LL * fact[n] * (1LL * invfact[k] * invfact[n - k] % MOD)) % MOD;
}
#endif

using namespace std;

int main() {
#ifdef HOME
  ifstream cin("A.in");
  ofstream cout("A.out");
#endif
  int i, n, m, v, p;
  ios::sync_with_stdio(false);
  cin.tie(0), cout.tie(0);

  cin >> n >> m >> v >> p;

  vector<int> a(n + 1);
  for (i = 1; i <= n; i++) {
    cin >> a[i];
  }

  sort(a.begin(), a.end());

  vector<ll> x(n + 1);
  vector<bool> vis(n + 1);

  auto check = [&](int pos) -> bool {
    int cur = a[pos] + m;

    int big = 0;
    for (i = 1; i <= n; i++) {
      if (a[i] > cur) {
        big++;
      }
    }

    if (big >= p) {
      return 0;
    }

    fill(vis.begin(), vis.end(), 0);
    int sz = n;

    for (i = n; i >= 1; i--) {
      if (i == pos)
        continue;

      if (a[i] > cur) {
        x[sz--] = 1e18;
        vis[i] = 1;
      } else {
        if (big < p - 1) {
          x[sz--] = 1e18;
          big++;
          vis[i] = 1;
        }
      }
    }

    for (i = 1; i <= n; i++) {
      if (i == pos || vis[i])
        continue;

      x[sz--] = cur - a[i];
    }

    x[1] = 0;

    pos = 0;
    ll sum = 0;

    for (int k = 1; k <= m; k++) {
      while (pos <= n && x[pos] <= k) {
        sum += x[pos];
        pos++;
      }
      if (sum + 1LL * k * (n - pos + 1) < 1LL * k * (v - 1)) {
        return 0;
      }
    }
    return 1;
  };

  int res = 0;
  for (int step = 1 << 18; step; step >>= 1) {
    if (res + step <= n && check(res + step) == 0) {
      res += step;
    }
  }

  cout << n - res;

  return 0;
}


Fixed submission:
#include <bits/stdc++.h>
#define lsb(x) (x & (-x))
#define ll long long
#define ull unsigned long long
#define uint unsigned int

#if 0
const int MOD = (int) ;

inline int lgput(int a, int b) {
    int ans = 1;
    while(b > 0) {
        if(b & 1) ans = (1LL * ans * a) % MOD;
        b >>= 1;
        a = (1LL * a * a) % MOD;
    }
    return ans;
}

inline void mod(int &x) {
    if(x >= MOD)
        x -= MOD;
}

inline void add(int &x, int y) {
    x += y;
    mod(x);
}

inline void sub(int &x, int y) {
    x += MOD - y;
    mod(x);
}

inline void mul(int &x, int y) {
    x = (1LL * x * y) % MOD;
}

inline int inv(int x) {
    return lgput(x, MOD - 2);
}
#endif

#if 0
int fact[], invfact[];

inline void prec(int n) {
    fact[0] = 1;
    for(int i = 1; i <= n; i++) {
        fact[i] = (1LL * fact[i - 1] * i) % MOD;
    }
    invfact[n] = lgput(fact[n], MOD - 2);
    for(int i = n - 1; i >= 0; i--) {
        invfact[i] = (1LL * invfact[i + 1] * (i + 1)) % MOD;
    }
}

inline int comb(int n, int k) {
    if(n < k) return 0;
    return (1LL * fact[n] * (1LL * invfact[k] * invfact[n - k] % MOD)) % MOD;
}
#endif

using namespace std;

int main() {
#ifdef HOME
  ifstream cin("A.in");
  ofstream cout("A.out");
#endif
  int i, n, m, v, p;
  ios::sync_with_stdio(false);
  cin.tie(0), cout.tie(0);

  cin >> n >> m >> v >> p;

  vector<int> a(n + 1);
  for (i = 1; i <= n; i++) {
    cin >> a[i];
  }

  sort(a.begin(), a.end());

  vector<ll> x(n + 1);
  vector<bool> vis(n + 1);

  auto check = [&](int pos) -> bool {
    int cur = a[pos] + m;

    int big = 0;
    for (i = 1; i <= n; i++) {
      if (a[i] > cur) {
        big++;
      }
    }

    if (big >= p) {
      return 0;
    }

    fill(vis.begin(), vis.end(), 0);
    int sz = n;

    for (i = n; i >= 1; i--) {
      if (i == pos)
        continue;

      if (a[i] > cur) {
        x[sz--] = 1e18;
        vis[i] = 1;
      } else {
        if (big < p - 1) {
          x[sz--] = 1e18;
          big++;
          vis[i] = 1;
        }
      }
    }

    for (i = 1; i <= n; i++) {
      if (i == pos || vis[i])
        continue;

      x[sz--] = cur - a[i];
    }

    x[1] = 0;

    pos = 0;
    ll sum = 0;

    for (int j = 1; j <= n; j++) {
      if (x[j] == 0)
        continue;

      ll k = min(x[j] - 1, 1LL * m);
      while (pos <= n && x[pos] <= k) {
        sum += x[pos];
        pos++;
      }
      if (sum + 1LL * k * (n - pos + 1) < 1LL * k * (v - 1)) {
        return 0;
      }
    }
    return 1;
  };

  int res = 0;
  for (int step = 1 << 18; step; step >>= 1) {
    if (res + step <= n && check(res + step) == 0) {
      res += step;
    }
  }

  cout << n - res;

  return 0;
}