Problem p02549 (C++) — buggy submission, verdict: Time Limit Exceeded

#include <bits/stdc++.h>

#include <ext/pb_ds/assoc_container.hpp> //required
#include <ext/pb_ds/tree_policy.hpp>     //required

using namespace __gnu_pbds;
using namespace std;

template <typename T>
using ordered_set =
    tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
// find_by_order(k)  returns iterator to kth element starting from 0;
// order_of_key(k) returns count of elements strictly smaller than k;
typedef long long ll;
typedef vector<ll> VL;
typedef vector<int> VI;
typedef pair<ll, ll> PLL;
typedef pair<int, int> PII;
#define pb push_back

#define F first
#define S second
#define SZ(a) int((a).size())
#define ALL(a) a.begin(), a.end()
#define fr(i, x, y) for (int i = x; i < y; i++)
#define frr(i, x, y) for (int i = x - 1; i >= y; i--)
#define inf 1e18 + 1
#define IOS                                                                    \\
  ios::sync_with_stdio(0);                                                     \\
  cin.tie(0);                                                                  \\
  cout.tie(0);
const int mod = 998244353;
ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }
ll power(ll a, ll b) {
  ll ans = 1;
  while (b != 0) {
    if (b & 1) {
      ans *= a;
    }
    a = a * a;
    b >>= 1;
  }
  return ans;
}
ll powerm(ll a, ll b) {
  ll ans = 1;
  while (b != 0) {
    if (b & 1) {
      ans *= a;
      ans %= mod;
    }
    a = a * a;
    a %= mod;
    b >>= 1;
  }
  return ans % mod;
}
VL dx = {1, 0, -1, 0};
VL dy = {0, 1, 0, -1};
// string to integer stoi()
// string to long long stoll()
// string.substr(position,length);
// integer to string to_string();

void solve() {
  ll n, k;
  cin >> n >> k;
  vector<PLL> arr(k);
  fr(i, 0, k) cin >> arr[i].F >> arr[i].S;
  VL dp(n + 2, 0);
  dp[1] = 1;
  dp[2] = -1;
  fr(i, 1, n + 1) {
    dp[i] += dp[i - 1];
    fr(j, 0, k) {
      ll x = min(n + 1, i + arr[j].F);
      ll y = min(n + 1, i + arr[j].S + 1);
      dp[x] += dp[i];
      dp[x] %= mod;
      dp[y] -= dp[i];
      dp[y] += mod;
      dp[y] %= mod;
    }
    dp[i] %= mod;
  }
  cout << dp[n] << endl;
  return;
}
int main() {
  IOS;
  ll t = 1, pp;
  // cin>>t;
  pp = t;
  while (t--) {
    // cout<<"Case #"<<pp-t<<": ";
    solve();
  }
  return 0;
}
/* stuff you should look for
 * int overflow, array bounds
 * special cases (n=1?)
 * do smth instead of nothing and stay organized
 * WRITE STUFF DOWN
 */
/*
recursion - matrix exponential

*/
// BITMASK:
// 1)When some constrall is of the order of 15-20, think of bitmask DP.
// 2)When some constrall is around 40, try out meet in the middle
// 3) See Strings,palindromes,prefix,suffix etc -> KMP,Z algorithm

Error: TLE

Fixed submission:

#include <bits/stdc++.h>

#include <ext/pb_ds/assoc_container.hpp> //required
#include <ext/pb_ds/tree_policy.hpp>     //required

using namespace __gnu_pbds;
using namespace std;

template <typename T>
using ordered_set =
    tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
// find_by_order(k)  returns iterator to kth element starting from 0;
// order_of_key(k) returns count of elements strictly smaller than k;
typedef int ll;
typedef vector<ll> VL;
typedef vector<int> VI;
typedef pair<ll, ll> PLL;
typedef pair<int, int> PII;
#define pb push_back

#define F first
#define S second
#define SZ(a) int((a).size())
#define ALL(a) a.begin(), a.end()
#define fr(i, x, y) for (int i = x; i < y; i++)
#define frr(i, x, y) for (int i = x - 1; i >= y; i--)
// #define inf 1e18+1
#define IOS                                                                    \\
  ios::sync_with_stdio(0);                                                     \\
  cin.tie(0);                                                                  \\
  cout.tie(0);
const int mod = 998244353;
ll gcd(ll a, ll b) { return b ? gcd(b, a % b) : a; }
ll power(ll a, ll b) {
  ll ans = 1;
  while (b != 0) {
    if (b & 1) {
      ans *= a;
    }
    a = a * a;
    b >>= 1;
  }
  return ans;
}
ll powerm(ll a, ll b) {
  ll ans = 1;
  while (b != 0) {
    if (b & 1) {
      ans *= a;
      ans %= mod;
    }
    a = a * a;
    a %= mod;
    b >>= 1;
  }
  return ans % mod;
}
VL dx = {1, 0, -1, 0};
VL dy = {0, 1, 0, -1};
// string to integer stoi()
// string to long long stoll()
// string.substr(position,length);
// integer to string to_string();

void solve() {
  ll n, k;
  cin >> n >> k;
  VL arr;
  fr(i, 0, k) {
    ll l, r;
    cin >> l >> r;
    fr(j, l, r + 1) { arr.pb(j); }
  }
  sort(ALL(arr));
  VL dp(n + 1, 0);
  dp[1] = 1;
  fr(i, 1, n + 1) {
    auto low = lower_bound(ALL(arr), i);
    if (low == arr.end()) {
      low--;
    }
    if (*low >= i) {
      if (low == arr.begin())
        continue;
      else {
        low--;
      }
    }
    ll x = low - arr.begin();
    fr(j, 0, x + 1) {
      if (i - arr[j] <= 0)
        break;
      dp[i] += dp[i - arr[j]];
      dp[i] %= mod;
    }
  }
  cout << dp[n] % mod << endl;
  return;
}
int main() {
  IOS;
  ll t = 1, pp;
  // cin>>t;
  pp = t;
  while (t--) {
    // cout<<"Case #"<<pp-t<<": ";
    solve();
  }
  return 0;
}
/* stuff you should look for
 * int overflow, array bounds
 * special cases (n=1?)
 * do smth instead of nothing and stay organized
 * WRITE STUFF DOWN
 */
/*
recursion - matrix exponential

*/
// BITMASK:
// 1)When some constrall is of the order of 15-20, think of bitmask DP.
// 2)When some constrall is around 40, try out meet in the middle
// 3) See Strings,palindromes,prefix,suffix etc -> KMP,Z algorithm