Problem p02591 (C++) — buggy submission, verdict: Time Limit Exceeded

/**Lucky Boy**/
#include <bits/stdc++.h>
#define FOR(i, a, b) for (int i = (a); i <= (b); ++i)
#define FORD(i, a, b) for (int i = (a); i >= (b); --i)
#define pb push_back
#define mp make_pair
#define F first
#define S second
#define maxn 270000
#define maxm 1000006
#define maxc 1000000007
#define pii pair<int, int>
#define Task ""
template <typename T> inline void read(T &x) {
  char c;
  bool nega = 0;
  while ((!isdigit(c = getchar())) && (c != '-'))
    ;
  if (c == '-') {
    nega = 1;
    c = getchar();
  }
  x = c - 48;
  while (isdigit(c = getchar()))
    x = x * 10 + c - 48;
  if (nega)
    x = -x;
}
template <typename T> inline void writep(T x) {
  if (x > 9)
    writep(x / 10);
  putchar(x % 10 + 48);
}
template <typename T> inline void write(T x) {
  if (x < 0) {
    putchar('-');
    x = -x;
  }
  writep(x);
  putchar(' ');
}
template <typename T> inline void writeln(T x) {
  write(x);
  putchar('\\n');
}
using namespace std;
const int mod = maxc;
int Mul(int a, int b) { return 1ll * a * b % mod; }
int Add(int a, int b) {
  a += b;
  if (a >= mod)
    a -= mod;
  return a;
}
int n, a[maxn], ans, sum[maxn];
vector<int> de;
void Dfs2(int u, int cur, bool left) {
  if (u == 1)
    return;
  cur = Mul(cur, u);
  if (left) {
    sum[u] = Add(sum[u], cur);
  } else {
    ans = Add(ans, Mul(u >> 1, Mul(sum[u ^ 1], cur)));
  }
  Dfs2(u >> 1, cur, left);
}
void Dfs1(int u, int cur, bool left) {
  cur = Mul(cur, u);
  if (u >= n) {
    int v = a[u];
    Dfs2(v, cur, left);
    de.pb(v);
    return;
  }
  Dfs1(2 * u, cur, left);
  Dfs1(2 * u + 1, cur, left);
}
int main() {
  srand(time(NULL));
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie();
  if (fopen(".inp", "r"))
    freopen(".inp", "r", stdin);
  // freopen(Task".inp", "r",stdin);
  // freopen(Task".out", "w",stdout);
  read(n);
  // cin >> n;
  n = 1 << (n - 1);
  FOR(i, 1, n) {
    // cin >> a[i + n - 1];
    read(a[i + n - 1]);
    a[i + n - 1] += n - 1;
  }
  FOR(i, 1, n - 1) {
    Dfs1(2 * i, i, 1);
    Dfs1(2 * i + 1, 1, 0);
    for (int &u : de) {
      while (u) {
        sum[u] = 0;
        u >>= 1;
      }
    }
    de.clear();
  }
  write(ans);
  // cout << ans;
  return 0;
}


Error: TLE

Fixed submission:

/**Lucky Boy**/
#include <bits/stdc++.h>
#define FOR(i, a, b) for (int i = (a); i <= (b); ++i)
#define FORD(i, a, b) for (int i = (a); i >= (b); --i)
#define pb push_back
#define mp make_pair
#define F first
#define S second
#define maxn 600005
#define maxm 1000006
#define maxc 1000000007
#define pii pair<int, int>
#define Task ""
template <typename T> inline void read(T &x) {
  char c;
  bool nega = 0;
  while ((!isdigit(c = getchar())) && (c != '-'))
    ;
  if (c == '-') {
    nega = 1;
    c = getchar();
  }
  x = c - 48;
  while (isdigit(c = getchar()))
    x = x * 10 + c - 48;
  if (nega)
    x = -x;
}
template <typename T> inline void writep(T x) {
  if (x > 9)
    writep(x / 10);
  putchar(x % 10 + 48);
}
template <typename T> inline void write(T x) {
  if (x < 0) {
    putchar('-');
    x = -x;
  }
  writep(x);
  putchar(' ');
}
template <typename T> inline void writeln(T x) {
  write(x);
  putchar('\\n');
}
using namespace std;
const int mod = maxc;
int Mul(int a, int b) { return 1ll * a * b % mod; }
int Add(int a, int b) {
  a += b;
  if (a >= mod)
    a -= mod;
  return a;
}
int n, a[maxn], ans, sum[maxn];
vector<int> de;
void Dfs2(int u, int cur, bool left) {
  if (u == 1)
    return;
  cur = Mul(cur, u);
  if (left) {
    sum[u] = Add(sum[u], cur);
  } else {
    ans = Add(ans, Mul(u >> 1, Mul(sum[u ^ 1], cur)));
  }
  Dfs2(u >> 1, cur, left);
}
void Dfs1(int u, int cur, bool left) {
  cur = Mul(cur, u);
  if (u >= n) {
    int v = a[u];
    Dfs2(v, cur, left);
    de.pb(v);
    return;
  }
  Dfs1(2 * u, cur, left);
  Dfs1(2 * u + 1, cur, left);
}
int main() {
  srand(time(NULL));
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie();
  if (fopen(".inp", "r"))
    freopen(".inp", "r", stdin);
  // freopen(Task".inp", "r",stdin);
  // freopen(Task".out", "w",stdout);
  read(n);
  // cin >> n;
  n = 1 << (n - 1);
  FOR(i, 1, n) {
    // cin >> a[i + n - 1];
    read(a[i + n - 1]);
    a[i + n - 1] += n - 1;
  }
  FOR(i, 1, n - 1) {
    Dfs1(2 * i, i, 1);
    Dfs1(2 * i + 1, 1, 0);
    for (int &u : de) {
      while (u) {
        sum[u] = 0;
        u >>= 1;
      }
    }
  }
  write(ans);
  // cout << ans;
  return 0;
}