Codeforces 1720D1 Xor-Subsequence (easy version)

Problem Link

The most apparant solution to this problem would be dynamic programming with $\mathcal{O}(n^2)$ complexity, where each $f_i$ would take $\mathcal{O}(n)$ time to calculate.

The key to this problem is the domain of $a_i$, which is only $200$. Since $a\oplus x\in [a-x,a+x]$, only the indice in range $i-400$ would possibly contribute to the computation of $f$. Therefore, we can narrow down the complexity to $\mathcal{O}(400n)$

#include
#include
#include
#include
using namespace std;
const int Maxn=3e5+10;
const int Maxm=512;
int a[Maxn],f[Maxn];
int n;
inline int read()
{
	int s=0,w=1;
	char ch=getchar();
	while(ch<'0'||ch>'9'){if(ch=='-')w=-1;ch=getchar();}
	while(ch>='0' && ch<='9')s=(s<<3)+(s<<1)+(ch^48),ch=getchar();
	return s*w;
}
int main()
{
	// freopen("in.txt","r",stdin);
	int T=read();
	while(T--)
	{
		n=read();
		for(int i=0;i<n;++i)
		a[i]=read();
		int ans=1;
		for(int i=0;i<n;++i)
		{
			f[i]=1;
			for(int j=max(0,i-512);j<i;++j)
			if(f[j]+1>f[i] && (a[j]^i)<(a[i]^j))f[i]=f[j]+1;
			ans=max(ans,f[i]);
		}
		printf("%d\n",ans);
		fill(f,f+1+n,0);
	}
	return 0;
}