E. Maximum Subsequence

https://codeforces.com/contest/888/problem/E

一半枚举，一半二分，orz

#include<bits/stdc++.h>
using namespace std;
#define ll long long
const int maxn=50;
ll  a[maxn],n,m;
int vis[40];
set<ll> s1,s2;
 
void dfs(set<ll>& s,int l,int r,int x){
	if(x==r+1){
		ll ans=0;
		for(int i=l;i<=r;i++){
			if(vis[i]){
				ans=(ans+a[i]%m);
			}
		}
		s.insert(ans%m);
		return;
	}
	vis[x]=1;
	dfs(s,l,r,x+1); 
	vis[x]=0;
	dfs(s,l,r,x+1);	
}
 
int main()
{ 
	scanf("%lld%lld",&n,&m);
	for(int i=1;i<=n;i++) scanf("%lld",&a[i]),a[i]%=m;	
	int mid=n/2;
	s1.insert(0),s2.insert(0);
	
	if(1==n){
		printf("%lld",a[1]);
		return 0;
	}
	
 	dfs(s1,1,mid,1); 
	dfs(s2,mid+1,n,mid+1); 
	
	set<ll>::iterator it,itt; 
	ll ans=0;
	for(it=s1.begin();it!=s1.end();it++){
		int temp=m-1-*it;
		ans=max(ans,*it);
		itt=s2.lower_bound(temp);
		if(itt==s2.end()) itt--; 
		
		if(*itt>temp)  itt--; 
		ans=max(ans,*itt+*it);
	}
	printf("%lld",ans);
	return 0;
}