Manthan, Codefest 19 (open for everyone, rated, Div. 1 + Div. 2) (1208F,1208G,1208H)

#include <iostream>
#include <sstream>
#include <algorithm>
#include <cstdio>
#include <cmath>
#include <set>
#include <map>
#include <queue>
#include <string>
#include <cstring>
#include <bitset>
#include <functional>
#include <random>
#define REP(i,a,n) for(int i=a;i<=n;++i)
#define PER(i,a,n) for(int i=n;i>=a;--i)
#define hr putchar(10)
#define pb push_back
#define lc tr[o].l
#define rc tr[o].r
#define mid ((l+r)>>1)
#define ls lc,l,mid
#define rs rc,mid+1,r
#define x first
#define y second
#define io std::ios::sync_with_stdio(false)
#define endl ‘\n‘
#define DB(a) ({REP(__i,1,n) cout<<a[__i]<<‘ ‘;hr;})
using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
const int P = 1e9+7, INF = 0x3f3f3f3f;
ll gcd(ll a,ll b) {return b?gcd(b,a%b):a;}
ll qpow(ll a,ll n) {ll r=1%P;for (a%=P;n;a=a*a%P,n>>=1)if(n&1)r=r*a%P;return r;}
ll inv(ll x){return x<=1?1:inv(P%x)*(P-P/x)%P;}
inline int rd() {int x=0;char p=getchar();while(p<‘0‘||p>‘9‘)p=getchar();while(p>=‘0‘&&p<=‘9‘)x=x*10+p-‘0‘,p=getchar();return x;}
//head
 
 
 
const int N = 3e6+10;
int n, a[N], vis[N], pre[N];
pii dp[N];
void add(int x, int p) {
	if (dp[x].x&&dp[x].y) return;
	if (dp[x].x==p||dp[x].y==p) return;
	if (!dp[x].y) dp[x].y = p;
	else dp[x].x = p;
	REP(i,0,21) if (x>>i&1) add(x^1<<i,p);
}
int main() {
	scanf("%d", &n);
	REP(i,1,n) scanf("%d",a+i);
	PER(i,1,n) add(a[i],i);
	REP(i,1,n-2) vis[i] = 1;
	int ans = 0;
	PER(d,0,21) {
		REP(i,1,n) if (vis[i]) {
			if (a[i]>>d&1^1) {
				pre[i] ^= 1<<d;
				if (!dp[pre[i]].y||dp[pre[i]].x<=i) vis[i] = -1;
			}
		}
		REP(i,1,n) if (vis[i]==1) ans |= 1<<d;
		REP(i,1,n) if (vis[i]==-1) {
			if (ans>>d&1) vis[i] = 0;
			else vis[i] = 1, pre[i] ^= 1<<d;
		}
	}
	printf("%d\n", ans);
}