Codeforces E. maximum subsequence value (greed + pigeon nest principle)

https://codeforces.com/contest/1365/problem/E
( Title link above ↑）
The question ：
from a Select a length of k The subsequence , Make the subsequence value Value is the largest . A subsequence value The value is 2^i And ,i Is the significant bit in binary form . If and only if there is no less than k-2 The binary of the number is 1 when , This bit is valid .
Answer key ：
1. When k<=3 when , Only a valid bit of a number is needed to produce a contribution . So you can choose three numbers directly ,O(n^3) Enumerate all the cases , obtain value The maximum of .
2. When k>3 when , When a bit is valid , It means at least k-2 This bit of the number is 1, This situation is obviously more difficult to meet . But if you choose three numbers from this subsequence , Then according to the pigeon nest principle, there must be a number that is valid . Sum up ,k>3 Of the situation value Value is less than or equal to the result of selecting three numbers value value .
3. therefore , Greedily enumerate and select three numbers .
An operation ：
0|0=0 0|1=1 1|0=1 1|1=1
so : value=ai | aj | ak
AC Code ：

#include <iostream> 
#include <algorithm>
using namespace std;
typedef long long ll;
ll a[505];
int main(){
    
	int n;
	cin>>n;
	for(int i=0;i<n;i++)
		cin>>a[i];
	ll ans=0;
	for(int i=0;i<n;i++){
    
		for(int j=i;j<n;j++){
    
			for(int k=j;k<n;k++){
    
				ans=max(a[i]|a[j]|a[k],ans);
			}
		}
	}
	cout<<ans<<endl;
	return 0;
}