506A - Mr Kitayuta the Treasure Hunter - CodeForces Solution

#include<bits/stdc++.h>
using namespace std;

#define rep(i, a, n) for(int i=(a); i<(n); ++i)
#define per(i, a, n) for(int i=(a); i>(n); --i)
#define pb emplace_back
#define mp make_pair
#define clr(a, b) memset(a, b, sizeof(a))
#define all(x) (x).begin(),(x).end()
#define lowbit(x) (x & -x)
#define fi first
#define se second
#define lson o<<1
#define rson o<<1|1
#define gmid l[o]+r[o]>>1

using ll = long long;
using pii = pair<int,int>;
using pll = pair<ll, ll>;
using vi = vector<int>;

constexpr int mod = 998244353;
constexpr int inf = 0x3f3f3f3f;
constexpr int N = 3e4 + 10;

int n, d, cnt[N], suf[N];
unordered_map<int, int> dp[N];

void _main(){
  cin >> n >> d;
  int x, y, m = 0;
  rep(i, 0, n){
    cin >> x;
    m = max(m, x);
    ++cnt[x];
  }
  suf[m] = cnt[m];
  per(i, m-1, 0){
    suf[i] = suf[i+1] + cnt[i];
  }
  dp[d][d] = cnt[d];
  int ans = cnt[d];
  rep(i, d, m + 1){
    for(auto &p : dp[i]){
      x = p.fi;
      ans = max(ans, p.se);
      rep(j, -1, 2){
        y = x + j;
        if(y <= 0)  continue;
        if(i + y > m) continue;
        if(y == 1){
          ans = max(ans, p.se + suf[i + 1]);
          continue;
        }
        dp[i + y][y] = max(dp[i+y][y], p.se + cnt[i+y]);
      }
    }
  }
  cout << ans << '\n';
}

int main(){
  ios::sync_with_stdio(0);
  cin.tie(0); cout.tie(0);
  _main();
  return 0;
}